What is the purpose of building foundations?

According to many sources, one of the purposes of building's foundations is "To distribute the weight of the structure over large area so as to avoid over-loading of the soil beneath." (Wikipedia) On the Wikipedia page on foundations, there is the following picture: So the foundation seems to consist of some type of "legs", with a larger surface area on the bottom. But if the point is to distribute the weight of the building over an area, why not put the bottom of the building in contact with ground? Isn't the bottom of the building itself a much larger contact area than the area the foundation can provide? In the picture, the bottom of the building does seem to rest at ground, so why do we require the foundation to have these large surface area rectangles at the bottom if the whole weight of the building is already resting on ground? Here is another picture from the same page: This seems to be some sort of lodge or a cabin, with stones used as foundation. Here it is evident that the building itself is raised above ground with the stones used as a contact to ground. But now the small stones suffer all the weight of the structure. Wouldn't they be under quite a heavy load over small contact area and possibly sink into the ground? Why not simply have the building's floor, a much larger contact area, rest on the ground?

That looks like a Pratt truss . These trusses have diagonals which go from the outer-top nodes to the inner-bottom nodes (i.e. they connect to the top chord on the node furthest from the center of the span, and to the bottom chord on the node closest to the center). This design means that the diagonals are under tension and the verticals are under compression. Another famous design is the Allan truss , which is the exact opposite: the diagonals go from the inner-top nodes to the outer-bottom nodes, which means that the diagonals are under compression and the verticals...

Down, Not Out Building foundations don't always have to spread the load out to a larger area. Sometimes the load only needs to be transferred to a stronger (harder) layer. This layer may be deep in the earth and have a softer layer on top of it. Layers Say you want to build a building in an area that has a lot of soft clay at the surface. This clay will not support much of anything (especially when it is wet). Underneath this layer is hard bedrock. You now have two choices: Add a basement to your building so that...

Biggest advantage is that when connecting to trailers, there are no bleeding issues. Imagine having to connect hydraulic pipes and remove the air bubbles... As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out. Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC. But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with...


How to calculate lever force when lever has uniformed distributed load?

We have a simple class 1 lever: $$\begin {array}{c} \text {5,000 kg} \\ \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \, \downarrow \\ ==================== \\ \hphantom {==} \triangle \hphantom {==============} \\ \vdash \text{1 m} \dashv \vdash \hphantom {======} \text{4 m} \hphantom {======} \dashv \\ \end {array} $$ The lever ($===$) is 5 m long. The fulcrum ($\triangle$) is 1 m from one end of the lever. The lever has a object sitting uniformly upon it weighing 5,000 kg. How do I compute the upward force that needs to be exerted at the end of the 1 m side of the lever to keep the lever stationary? $F = (W \times X)/L$ is simple when the weight is applied at the very end of the lever. But what happens if the weight is distributed along the lever? Our final goal is to tether the free end (on the 1m side) to keep the lever level and we need to know how strong the tether should be.

Answer: Yes , this is exactly how it is done in the real world. What you have described is what I do in my job to check systems in CAD. Since you indicated you would like me to step through my design process, I have detailed it below. Note that most of this does not involve CAD. CAD is invaluable, but only if you are prepared to get out the pencil and paper first. Also note that this is just my design process, it is by no means the only way to go about things. Preparation When I begin any...

Since the mass is 5k kg and the lever is 5m, this makes it quite easy to simplify because it is exactly 1k kg per m. The leftmost 2k kg (2m) of the mass has its center of mass exactly above the fulcrum so can be ignored as it provides no contribution to the moment. This leaves 3k kg (3m) spread from 1m to 4m on the right side. The center of mass will therefore be at 2.5m. Now it's super-simple, assuming you want the moment when the lever is level (i.e. when gravity is pulling straight down, perpendicular to...

OP injection molding tag is correct. OBall uses injection molding and plastic welding. The OBall is the invention of David E. Silverglate. Toy Ball Apparatus with Reduced Part Count Reduced image from Kids II . It consists of four identical, flat, injection molded, pentagon and hexagon shapes with circular (or elipitical) holes, which are shaped and plastically welded into spheres. Pentagon and hexagon edges are the same size and individual connected circles are only connected along one edge. The four shapes are clearly shown in colors above and from the patent. Solid lines on each part are hard connections, while...

In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller: I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps. Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and is worse on the outer edge of the propeller which is moving faster through water than the inner area. I doubt these spheres have any effect on cavitation at all. It is either a...


How do you mix quantitative asset allocation with qualitative views?

Usually in asset allocation you have a quantitative approach (which can be from example mean-variance), but you (or you and your firm) also have a more qualitative approach given market-conditions, economic outlooks, or tactical indicators. Hence, you will eventually come up with 2 allocations, the ones strictly dictated by the numbers $w^*$ which is the result of your quantitative algorithm and the one you have in mind from your personal expectations $\bar{w}$. What are common the ways $f$ to mix them together such that $w=f(w^*,\bar{w})$ is your "final" allocation?

There are some cases where you can blend your portfolios using weights directly. One case involves corner portfolios . In this case a linear combination of weights is also efficient. Another case is where you can treat the two separate weights you have produced each as distinct portfolio under the assumption that the correlation between these portfolios is relatively stable. In this scenario, the problem reduces to a two-asset portfolio optimization problem (each asset is simply the linear combination of weights produced via your two methods). The other class of methods involves blending via the expected returns. If you arrived...

I have worked on this topic extensively (pricing and calculating IV in production) and believe can offer an informed opinion. First of all Mathworks - the company that creates Matlab is not a trading firm so you should probably not rely on their advice so much. There are few closed form options pricing models, and all have practical shortcomings. Barone-Adesi and Whaley (please correct my spelling of last names as I'm typing from memory) model is simple approximation for American options but is unfortunately not very accurate, and does not deal with dividends. Roll-Geske-Whaley deals with dividends, but not very...

I think you're overlooking a third explanation: Nobody that found a successful technique to generate alpha has published it. I can think of the following causes: If you're an academic, why share your brilliant idea? These techniques require a lot of data and financial data can be expensive, researches that work at firms that have access to this data don't share their findings with the public. Academics did find a lot of signals already the old fashioned way. Despite this, fancy techniques such as AAD and Reinforcement Learning are discussed publicly. These methods don't generate any alpha however.

I've worked at a hedge fund that allowed GA-derived strategies. For safety, it required that all models be submitted long before production to make sure that they still worked in the backtests. So there could be a delay of up to several months before a model would be allowed to run. It's also helpful to separate the sample universe; use a random half of the possible stocks for GA analysis and the other half for confirmation backtests.


If a company agrees to pay travel cost for a job interview, is the promise binding and enforceable?

There is a question on workplace.SE about a company which offered to pay travel costs for a job interview, but canceled the return ticket after ending the interview - Potential Employer Cancels Return Flight . Generally, when arranging a job interview which requires the candidate to travel, it is common for the potential employer to pay travel cost and lodging for the candidate. In practice, the employer will ususally offer this, often in writing, but not write up a formal contract or similar document. In that situation, can the employer later renege on the promise to pay costs? In particular, can they renege after the candidate has begun their journey, thus saddling the candidate with the travel cost? My thoughts: On the one hand, a simple one-sided promise is usually not binding, as a binding agreement requires both sides to give something (called consideration in Common law). On the other hand, there is the notion of detrimental reliance , which appears to fit here. Also, one could argue that the candidate does provide something in exchange, namely their presence and willingness to attend the interview (basically, the company trades travel costs for having the candidate present for the interview). So, is the promise to pay the candidate's cost enforceable or not? Im interested in any jurisdiction, particularly European and US.

This aspect (and many others) of contract law is applicable in the US and various countries of the EU. can they renege after the candidate has begun their journey, thus saddling the candidate with the travel cost? No. The company would incur breach of contract. There is no need for a formal contract. The candidate only needs to prove that the company agreed (in writing, orally or clearly through its conduct) to cover or reimburse those expenses and that this elicited a meeting of the minds . The agreement would be void if the candidate incurred the expenses despite knowing...

The Plan For A Clearly Guilty Client Without Bargaining Power This question underestimates how much of a criminal defense lawyer's work involves sentencing rather than a determination of guilt or innocence. Suppose as the OP does that the prosecution can easily prove beyond a reasonable doubt that your client is guilty, you client has no plausible defenses, and the prosecutor won't budge on a plea. As a criminal defense lawyer, you may well advise your client that there is no percentage in fighting guilt on the charges, and have your client plea guilty. This prevents the prosecutor from spelling out...

No Parliament is sovereign : Parliamentary sovereignty is a principle of the UK constitution. It makes Parliament the supreme legal authority in the UK, which can create or end any law. Generally, the courts cannot overrule its legislation and no Parliament can pass laws that future Parliaments cannot change . Parliamentary sovereignty is the most important part of the UK constitution.

There Is Little Pressure To Improve Contract Drafting One piece of the puzzle is tradition and habitual writing style. A customary writing style for legal documents evolved over time, some of those conventions have evolved overtime (e.g. "Party of the first part, party of the second part"), others have not. Archaic vocabulary is slowly being weeded out, but again, it is almost never the goal of someone drafting a contract to reduce the reading level of the document. As long as the judge, or other lawyers, know what it means, it is an aesthetic stylistic issue more than it is...


Consent to searches: Who wins "my-word-against-yours?"

Hypothetical Officer testifies citizen consented to be searched. Citizen denies consenting to be searched. There is no other evidence (that weighs net in favor of either party). Question Whom does the judge believe? Clarifying Discussion from previous comments... Point The testimony of the officer and the citizen would be more complete than just the officer saying "The defendant consented to be searched" and the defendant saying "I did not consent to be searched". Each would give a narrative of the situation that could be cross-examined. This would give the judge more information on which to base a judgement of trustworthiness. Response Although everything you say is true, to put all that information into the question would miss the point. Which is: What is the standard of proof (for consent to searches)? And is their any " weight " advantage (in practice) for testimony from either side (all other factors being equal). For the purpose of this question, please assume all the other factors (the ones you mention and all the others) cancel out each other. Imagine both narratives agree except in one version the citizen gives consent. In the other he does not.

Assuming U.S. Jurisdiction: In the case of The People vs. Hansel and Gretel Holzfaller: Ms. Gretel Holzfaller is charged with the following: 1 Count of Murder in the First Degree (Murder of Ms. Witch Hazel) 1 Count of Grand Larceny (Theft of precious metals and jewels from Ms. Hazel) 1 Count Petty Theft (Theft of Candy) 1 Count of Vandalism (Bite marks left in Gingerbread Masonry) 1 Count Trespassing Mr. Hansel Holzfaller is charged with the following: 1 Count of Accessory to Murder 1 Count of Grand Larceny 1 Count Petty Theft 1 Count of Vandalism 1 Count Trespassing Ms....

Most leases have a provision allowing a landlord to make entry without notice in an emergency, but the better course of action, as noted in a comment by @BlueDogRanch, is to call the police and ask them to make a "welfare check." You would ordinarily be permitted to cooperate with police by unlocking doors in furtherance of their welfare check. The police are trained to do this properly in a way that properly balances the need to aid someone who is sick or ill, the need to preserve evidence if there was a death or crime that needs to be...

This is not a simple question. There are hundreds of page treatises on consent - explicit, implied, obtained by trickery, revoked, coerced by show, coerced by intimidation. If it ever was just a he said (s)he said situation, that certainly does not mean that the testimony of those two individuals are all that's considered to determine (1) that the prosecutor failed to meet it's burden; nor does it mean (2) that the consent was (legally) given. None of these answers (nor this answer) could possibly adequately address the complexity of the analysis a court will endeavor to undertake when evidence...

There is no legal obligation of the police or any of its officers to reimburse you. It comes within a police powers exception to the 5th Amendment obligation to provide compensation for takings. Sometimes a government will compensate someone even though it has no legal obligation to do so, but this is unlikely to happen. A petition for certiorari from 10th Circuit decision of Lech v. City of Greenwood Village (10th Cir. October 29, 2019) recaps a lot of the relevant law and arguments for changing it (the petition was subsequently denied by the U.S. Supreme Court). The question presented...


Can I sue a restaurant for serving me meat in vegetarian dish?

I am a vegetarian for religious reasons. I was at Chipotle on Saturday and ordered a sofritas rice bowl which is "vegan approved". To my surprise, I found a chunk of chicken in my bowl when I was halfway through eating it. Made me feel sick to the stomach realizing that I may have eaten chicken already. Went back to the Order desk and showed the server meat in my bowl. They immediately offered a refund and asked if I wish to file a complaint. The manager brought out a complaint form and filled for me. Gave me a copy as well. I did not take any refund. just walked out, I was very upset. My question is: Can I take a legal action against Chipotle?

Yes you can, and you can even include "editorials or subjective content". However, if you include factual statements, or words that imply factual statements, the company could claim that they are false, and therefore defamatory. Indeed they might claim that in any case. If you make no false statements of fact, they should not be able to win a defamation suit, but you might need to spend time and money defending yourself if they choose to sue. The detailed rules on defamation vary by jurisdiction, in the US by state. But in no US state can defamation be found against...

While there is an act that President Trump and his supporters are citing, titled the Presidential Records Act (PRA) , to accuse House Speaker Pelosi of breaking federal law, it is important to understand what actions the law accounts for. The act mandates that the President of the United States preserve records and other laws governing the federal government. This serves as a form of checks and balances to prevent the president and his advisers from shielding documentary information from public view. The act is fairly new, as it was passed in 1981. It is important to realize that this...

Yes, it's illegal You are missing something terribly important: The package might not be your property [yet]. In any way, it is not in your possession, while it is in the hands of the postal service! The contents of the package started fully owned by the sender and were entrusted to the postal service to deliver it. This entrustment is (contractually) defined as the time it is handed to the postal service, but the postal service does not gain any ownership. They do however have insurance on the parcel (to some degree), as they are liable for the loss of...

An essential component of your contract with them is that they will provide you with food free of animal stuff (the exact nature of "vegan approved" may be up for debate, but actual meat should not be included). So they breached their contract with you, and you might sue them for breach of contract. The case of Gupta v. Asha (orders were mixed up) could be useful in this matter. In this case, Hindus were served samosas containing beef, despite repeated assertions that they were vegetarian. Plaintiffs sued, the case was dismissed, and the appeals court deemed that the lower...


How much clearance does a car need when turning a corner?

I am contemplating buying a new car. However, the approach to the underground garage in my apartment has a 90 degree frustrating turn. Given the dimensions of the approach and car, what is the max turn circle for the car to fit the garage and turn? given Ackerman steering and the overhanging front part of the car I believe you can use Pythagoreas' theorem to get R min and R max. delta R should be less than the shortest path in the pathway, ie 2.5m. unfortunately the result does not seem plausible. feedback would be greatly appreciated.

Please note : I'm not a building designer by trade, but I have had to investigate related questions for other reasons. I'll let you in on a dirty secret about sanitation lines within buildings - The biggest concern is not about how fast things are flying, it's about maintaining air pressure and providing adequate ventilation. This guide presents a bit of a historical background to tall building design with respect to sanitation system design. This presentation provides some more recent perspective and goes into some of the research that is refining current building standards. The traditional viewpoint is that waste...

Those are counterweights . They work exactly the same as those lead counterbalance weights on the wheels of your automobile. If they left those out, then those connecting rods and bearings would create an out-of-balance condition, and the wheels would vibrate at higher speeds. That could very well damage the wheels. But as a couple of others have nicely pointed out, the violent shaking could derail the train .

It depends on how steep the hill is. On a slight hill, the energy added by gravity is still not enough to overcome rolling friction and air resistance, so the car still needs power to maintain speed. On a steeper hill, the two may balance out, so no power is used, and no power is generated. On a hill that's steep enough to require braking to control the speed, the car recovers energy. It's called regenerative braking. If the car is going too fast, applying the brakes turns the motor into a generator and charges the battery.

To slightly generalize I'll reform the question slightly. A ridged 2-D body (car) has a line $l$ that moves with it. The car can be linearly transformed as long as the instantaneous center of rotation lies along $l$ at least distance $R$ away from a point $c$ that also moves with the car. In this case point $c$ lies in the center of the rear axle and $l$ lies on the rear axle. Now imagine the car's domain is limited to a quarter plane with edges $A$ and $B$. It initially is placed against $A$, far from $B$ with $l$...


Would it be legal to supply an item that deliberately stops working after the guarantee expires?

I have TV that failed about a week after the 3 year guarantee expired. Obviously this is annoying, and I'm back to using a 15 year old TV that I dug out from the attic. Realistically, this failure so close to expiry of the guarantee is probably just a co-incidence, but it did get me thinking, the TV is a 'smart TV' with lots of electronics inside. It wouldn't be difficult to deliberately design it to stop working after exactly 3 years. If a manufacturer deliberately did this (and didn't mention it in their sales literature) would this be lawful? This is slightly different from 'planned obsolescence' whereby spare parts aren't available, or an item can't be disassembled without destroying it, I'm talking about something very similar to buying a software license - where a fixed fee up-front allows you to use a product for exactly 12 months. In practice, when I buy a physical item I regard the guarantee as the minimum period the item should work for with the expectation that with a bit of luck it should work for a lot longer. Could a manufacturer lawfully sell a TV/kettle/electric drill deliberately designed to fail after a fixed period?

australia Deliberately rendering the TV non-operational through software would appear to be malicious damage. Under the Australian Consumer Law, one of the statutory guarantees is that a product must be “durable”. Manufacturer’s can offer explicit warranties in addition but these in no way impact on the statutory guarantees. What durable means depends on the product and the price paid for it. For a low end TV, 3 years might be durable enough although it’s arguable, but for a high end TV it wouldn’t be. For whitegoods, 10 years would be appropriate. A similar period would be appropriate for a car....

A "land contract" is not a way of renting property, it is a way of purchasing property on an installment basis without bank financing. It is Ohio's version of what in some other places is known as "contract for deed". See "What is a Land Contract in Ohio" and "How Land Contracts Work" The actual law is Section 5313 . In a land contract, the buyer has equitable but not legal title. The buyer normally pays all taxes and fees, and is responsible for maintaining the property, just as if s/he has bought the property. But if the buyer defaults,...

Short Answer Applicants are required by military regulation to have a percentile score on a standardized test called the ASVAB that is 31 or more, which is roughly comparable to an IQ score on the Stanford-Binet scale of a little bit less than 92, for high school graduates seeking to enter the Army or Navy (other services have more strict requirements and applicants with only a GED or to the national guard must have an ASVAB score in the 50th percentile which is equivalent to an IQ of 100). But, U.S. law allows the Department of Defense to allow people...

In the united-kingdom , the legal basis flows from the Sanctions and Anti-Money Laundering Act 2018 . This allows regulations to be made for particular sanctions regimes, in this case The Russia (Sanctions) (EU Exit) Regulations 2019 and its several amendments, including three revisions this year. Under those regulations, the Secretary of State may designate individuals who are then subject to particular sanctions. A consolidated list is available for all individuals and organizations who have been named under any of the regulations, not just the Russia one. Working backwards from the list, we see such names as Vladimir Putin, who...


What is the most efficient means of warming a building with a high ceiling?

Consider a large auditorium, a church, or some other very large, essentially one roomed building with a high ceiling. Suppose that the building has many entrances which enable cold air in/hot air out and traffic in and out of the building is unavoidably large. I would imagine that any attempts to control the temperature of such a large building would be very inefficient in terms of energy and cost, particularly because warm air rises. Assuming that it's very cold outside and we're interested primarily in keeping the building warm at the ground level so that it is comfortable for humans to work and interact, what is the best method to keep such a large, essentially one roomed building with a high ceiling warm when outdoor cold air exposure is frequent and unavoidable? When I say "best," I'm interested in balancing energy, maintenance, and monetary costs over the life of the building.

civil-engineering building-physics energy hvac

There are a few main reasons why suspension bridges aren't used for railroads. The main reason is that suspension bridges are typically used where very long spans are needed. Trains are very heavy, especially when compared to lanes of highway traffic. This means that long spans require very strong support structures, which in the case of suspension bridges are cables and towers. The second reason goes along with the first; trains cause high dynamic loads as they move along the rail. This can increase the vertical loads by 30%. Third is that trains don't really have suspensions, especially freight trains....

When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat . The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or the power at which the plant was operating before shutdown. This can be large for commercial plants, because they typically operate at or very near capacity and the power companies bring coal or natural...

Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....

Your question sort of has two parts: How to supply heat, and how to keep it in. Large open rooms with a high ceilings are most efficiently warmed with radiant ceiling heat. Warm air rises, which renders forced-air systems inefficient because the pumped heat ends up at the ceiling and the coldest part of the room is near the floor where you actually want the heat. Radiant floor systems are limited to about 87F because they are in contact with occupants, and so their peak output may not be enough to keep the space comfortable. They also lose more heat...


Laptops in the dark and miscalculating bats: how dangerous is that for rabies?

A few nights ago, during a township-wide power failure, I was using my laptop outside in the dark. Because mine was one of very few lights for many acres, a more than usual number of moths and other insects were flying around in the light it was emitting, but unfortunately I didn't think anything about this from a bat's perspective. Suddenly (and incredibly silently) something flew into my face, completely blocking my vision momentarily. I thought at first that it was a giant moth, but it was too fast for a moth, and besides, it flew away immediately. But it took a few moments to realize that it was the wing of a flying bat that had basically hit me in the face. I had no scratches, no blood, didn't feel any fluid when it hit me, and I washed myself well. About 15 years ago, I was exposed to the saliva of a known rabid animal and had completed a post-exposure prophylaxis series of 5 vaccines as well as getting a rabies immune globulin shot, so I wasn't overly concerned. * I know that any significant first exposure to a potentially rabid animal, or a possible rabies-vector (bats, raccoons, skunks, foxes, etc.) merits a visit to an emergency room and immunization. But what about this situation? Are there clear guidelines about post-exposure prevention in people who have been previously immunized? * From that night on, when using my laptop outside at night, I make sure to turn on an outdoor light brighter than my computer screen.

I think a missing bit of information that might help you get a better sense of this practice is: steroids are miracle drugs. OK, that was in jest - no miracles here. Truth be told, though, if there is a single class of drugs that has added more quality-adjusted life-years to human history than any other, steroids must be competing with just a few antibiotic classes for that title. To make clear what we’re talking about, the term “steroid” as a label for drugs generally refers to glucocorticoids (GCs) - drugs that act like cortisol, an endogenous steroid hormone. Commonly...

This event occurred in the US. In the US, enzootic (dog-to-dog) canine rabies virus has been virtually eliminated through vaccination and stray control programs, making wild animals the primary concern. It is quite true, as @EMT_Jedi stated, that rabies is usually caused by an animal's saliva, usually introduced by a bite (e.g. rabid cats, raccoons, etc.) However, this is not true of bat-related rabies. There is reason to be vaccinated after any strange contact with a bat. The most dangerous and common route of rabies exposure is from the bite of a rabid mammal. An exposure to rabies also might...

In general, the benefit of flu shots is to the general population. Taking the cited value of 40%-60% from the CDC, we can say that it might be a coin toss for you personally to be protected from influenza by the shot. However, if many thousands of people take the flu shot, even if it only works on 40% of them, it helps protect the remaining population from the spread of the virus (AKA herd immunity). This is especially important to those within the population who cannot access the shot or cannot receive it due to a preexisting medical condition....

The short version is that in 2016 the polio vaccine changed. A more thorough explanation requires some background on the immunology of polio and its vaccines, which is not straightforward. Polio virus is usually harmless, it reproduces in the gut and spreads through a fecal-oral route. In ~99% of infections it only causes mild diarrhea. In the remaining 1% of cases, however, it gets into the bloodstream and from there enters nerves, causing paralysis and/or respiratory failure. There are two types of vaccine: inactivated polio vaccine (IPV) and oral polio vaccine (OPV). IPV is a shot of killed virus particles...


How did mechanical engineers work before SolidWorks?

I am studying a bunch of stuff related to mechanical engineering and am considering an eventual switch to the field after taking the necessary classes. I'm older, and I took drafting classes in high school and part of college. I'm familiar with the standard drafting procedure with paper, as well as AutoCAD , and these days SolidWorks . It's a shame they don't teach paper drafting much anymore! But I am left with a question I don't have the answer to, nor the resources to answer it. These days it seems most mechanical engineers (at least the ones I know) are really performing the role of both the drafter and the engineer. Prior to 1995 when SolidWorks came about, what was the duty of the mechanical engineer? If we go even further back before CAD was invented, what duties did a mechanical engineer perform?

cad drafting software engineering-history

Water meets the low compressibility requirement, but there are many other considerations in the design of a hydraulic system: Boiling point/vapor pressure: If the system warms up during operation, the fluid may boil, which results in high compressibility and thus decreased effectiveness of the hydraulic system. Hydraulic fluid has a higher boiling point than water to help combat this. Related to this is the concept of vapor pressure. Hydraulic systems often involve small orifices, which can cause cavitation (localized boiling). This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region....

Meet Vladimir Shukhov , a Russian architect who first developed hyperboloid structures. He was born in 1853, died in 1939, and created over 200 hyperboloid structures in the intervening years. He was the reason hyperboloids gained the popularity that they did. His first design, the first hyperbolic structure ever, was the Shukhov Tower in Polibino , pictured here : Another tower also bears the name of Shukhov, and it achieved great fame, too. Shukhov also built the Adziogol Lighthouse . In total, Shukhov designed and built 200 hyperboloid structures. He died in 1939, which could one reason for the decline...

The problem is that the formula $P=I\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\ V\ \cos(\phi), $$ where $\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...

At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...


What are the advantages/disadvantages of these approaches to deal with volatility surface?

I would like to know if someone could provide a summarized view of the advantages and disadvantages of the approaches on the volatility surface issues, such as: Local vol Stochastic Vol (Heston/SVI) Parametrization (Carr and Wu approach)

The volatiltiy surface is just a representation of European option prices as a function of strike and maturity in a different "unit" - namely implied volatility (while the term implied volatility has to be made precise by the model used to convert prices (quotes) into implied volatilities - for example: we may consider log-normal vols and normal vols). Volatility is often preferred over prices, e.g., when considering interpolations of European option prices (although this may introduce difficulties like arbitrage violations, see, e.g., http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1964634 ). A local volatility model can generate a perfect fit to the implied volatility surface via Dupire's...

In general there are two basic ways to make money out of your option pricing models: Sell side (market maker, risk neutral): You use these models to calculate your greeks to hedge your portfolio, so that you live on the spread. Buy side (market/risk taker): You use your model to find mispriced options in the market and buy/sell accordingly. (A third possibility would be to write fancy books and papers about these models and get rich and/or tenure this way ;-)

Your questions about contango in VIX futures have close analogies in options too. The Black & Scholes model suggests that all time frames and all strikes should have the same implied volatility, but they don't. I think one of the reasons is that the B&S model assumes that stock returns are distributed in a normal (gaussian) distribution, but the actual returns don't match a gaussian distribution all that well. For example the actual occurrence of big crashes / gains is much more likely than a normal distribution would predict. Since crashes do occur people are willing to pay what the...

There are some cases where you can blend your portfolios using weights directly. One case involves corner portfolios . In this case a linear combination of weights is also efficient. Another case is where you can treat the two separate weights you have produced each as distinct portfolio under the assumption that the correlation between these portfolios is relatively stable. In this scenario, the problem reduces to a two-asset portfolio optimization problem (each asset is simply the linear combination of weights produced via your two methods). The other class of methods involves blending via the expected returns. If you arrived...


Do Legal Documents Require Signing In Standard Pen Colors?

I have a question as to whether or not legal documents signed in non standard pen colors (Anything other than blue or black) are valid. I carry a purple pen around that use for everyday writing tasks, and when I was going to sign a document, someone told me that writing in purple is not valid on legal documents. If the document does not specify that a certain pen color be used, is this true?

united-states contract-law new-york-state signature

at what point can you just leave? Is it always technically illegal in the UK to leave without paying the bill? Probably depends on what you mean with just leaving. If just leaving translates I haven't paid and I won't pay (because of the hassle with the card) then that's probably Making Off Without Payment, section 3 Theft Act 1978 (Thanks @bdsl). Could the restaurant just force you to wait until close of business if necessary? What if they still hadn't fixed the payment system by then? I don't think a restaurant can physically detain you. Not even the 45...

No, Specific Ink Colors are not Required That is not correct. If the purple will not photocopy well, the other party might reasonably ask for a color that will. But a signature is normally only evidence of agreement to the provisions, and it is the agreement that is legally important. The color of the ink used does not change the agreement. It is normal to expect a signature to be in a permanent ink. A signature in pencil or erasable ink might be legal, but the other party will not want to accept it, and it would be reasonable to...

Trump was an officer of the government, and Twitter wasn't. The First Amendment forbids the government and its agents from viewpoint discrimination, but private companies are not bound by it and can discriminate as much as they please. (There was a question as to whether such discrimination might affect whether the company enjoys a shield from liability under 47 USC 230 , but even so they have the right to block and censor as they wish if they are willing to risk that liability.)

If you are not a member of the Bar of Maryland, you may not "practice, attempt to practice, or offer to practice law in the State unless admitted to the Bar." Maryland Business Occupations and Professions § 10-601 . "Practicing law" includes "representing another person before a unit of the State government or of a political subdivision." Maryland Business Occupations and Professions § 10-101 . "[A] person who violates § 10-601 of this subtitle is guilty of a misdemeanor and on conviction is subject to a fine not exceeding $5,000 or imprisonment not exceeding 1 year or both." Maryland Business...


In my firm we are beginning a new OMS (Order Management System) project and there is a debate whether we use Quickfix or we go for a professional fix engine? Because there is a common doubt that QuickFix is not enough fast and obviously we will not get any technical support. I heard that in BOVESPA it has been used for a while. They are changing it with a paid one now. Well that is enough for me. If they use it I can use it. Should I choose a professional one over QuickFix? Is it not good enough?

1. Determine Factors Economically, the use of factor models can be either motivated using the ICAPM or the APT . Although there are some theoretical differences between the model, for empirical and practical work these differences are irrelevant. In the end, both models stipulate that returns and expected returns are linear functions of the factors: $$ r_{i,t} = \alpha_i + \sum_j \beta_{i,j} F_{j,t} + \epsilon_{i,t} \quad (1)$$ $$ \mathbb{E}[ r_{i,t}] = \lambda_o + \sum_j \beta_{i,j} \lambda_j \quad\quad\quad(2)$$ where $F_{j,t}$ is the factor surprise of factor $j$ at time $t$ and $\lambda_j $ is the factor risk premium of factor $j$....

In order to answer your question (for you) you would need something to compare to . You would need numbers to know if it is slower/faster, how much, and if it will impact your system overall. Also knowing your performance goals could narrow down the options. My advice is to take a look at your overall architecture of the sytem you have or intend to build. To just look at QuickFIX is rather meaningless without the whole chain involved in processing information and reacting to it . As an example, say QuickFIX is 100 times faster than some part (in...


What is the purpose of a disclaimer like "this is not legal advice"?

If someone who is not a lawyer is giving out legal advice, does it make any difference if they include a disclaimer along the lines of "this is not legal advice"? For the purposes of this question, I assume legal advice means one party instructing another party on how to comply with laws (like a consultant might do). Do they disclaim some kind of liability? Do they merely avoid suggesting they are lawyers when they're not?

european-union germany lawyer disclaimers unlicensed-practice

There isn't any indication in that news story that the disabled son was anywhere nearby. I agree the situation you describe sounds like a legitimate use of the placard, but it seems in this situation, the placard was being used in a manner totally unrelated to the transport of a disabled person. My guess is that the cops cited her because the son wasn't in the car, and was not inside the establishment at which she parked. California code has this to say: A person to whom a disabled person placard has been issued may permit another person to use...

Public nudity is illegal in New York as it is in almost every U.S. jurisdiction. And, if the police had arrested the nude protesters, the arrests probably would have been upheld in court, because a ban on nudity in public would probably be viewed as a time, place and manner restriction on the freedom of speech which is constitutionally valid. For example, the Ninth U.S. Circuit Court of Appeals recently upheld San Francisco’s public nudity ordinance in Taub v. City and County of San Francisco (2017). As the court in that case explains in a factually very similar case: Public...

Not going to hold up. Dutch Supreme Court confirmed 2012-09-21 in LJN BW6135 that arbitration is still covered by the the right to an independent judge, as established in Golder v UK , ECHR 1975-02-21, nr. 4451/70. Stack Exchange can't decide the rules themselves. (The Dutch case confirms that sector-wide arbitration is in fact legal, with regard to a standard arbitration clause commonly used in the Dutch building sector. The arbiter was found to be independent in that case precisely because they weren't picked by the builder involved.) The GDPR is only indirectly relevant, but the fact that it's mentioned...

In most jurisdictions, practicing law without a bar license is a serious offence, which, inter alia, is the primary reason why a non-lawyer would use this disclaimer. Lawyers also use this disclaimer to avoid any 'constructive implication' of attorney-client relationship.


While browsing a local news site, I stumbled upon this picture of a broken bus door. The glass was fractured due to a rock impact. As far as i know there are no heating elements on the window section in the picture. Now, what interests me is the apparent periodicity and structure of the fracture pattern. Are periodic fractures something commonly encountered in tempered glass? Is the mechanism that generates such fractures understood? Is it a consequence of the manufacturing process? Perhaps the way the glass is affixed to the door? Thanks in advance

It's an artifact of production - in particular, the construction of the conveyor belt of the machine performing the quenching process. The pattern can be observed through a polarizing filter in the undamaged glass: source and it's a result of the structure of the conveyor belt through which the surface of the glass is cooled to generate the stress that gives it the special properties: source The contact area of the glass with the conveyor is small enough that it doesn't negatively impact the process - all throughout the surface the stress is introduced in sufficient amount, but the amount...

The problem is that the formula $P=I\ V$ is correct when dealing with DC circuits or with AC circuits where there is no lag between the current and the voltage. When dealing with realistic AC circuits, the power is given by $$ P=I\ V\ \cos(\phi), $$ where $\phi$ is the phase difference between the current and the voltage. The unit kVA is a unit of what is called 'apparent power' whereas W is a unit of 'real power'. Apparent power is the maximum possible power attainable when the current and voltage are in phase and real power is the actual...

Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue. One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book " Wind Energy - The Facts ". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence. Open land also...


Derivation for bridge natural frequency estimate in Eurocodes

The Eurocodes gives the following equation for estimating a "simply supported bridge subject to bending only"*: $$n_0 = \frac{17.75}{\sqrt{\delta_0}}$$ Where $n_0$ is the natural frequency in hertz $\delta_0$ is the deflection at mid-span under permanent actions in mm The equation is seemly plucked from thin air, and there is no explanation as to where the constant 17.75 comes from. As an engineer I'm loath to use a formula I don't understand, but more than that it would be helpful to learn the fundamentals behind it so that I can see if it can be altered to work with other support conditions. Can anyone provide a derivation / fundamental origin to this relationship? *Full reference is: EN 1991-2:2003 6.4.4 [Note 8] (Equation 6.3), if that helps.

The picture, below, of the exaggerated long section of the Channel Tunnel was taken from Wikipedia . Full-sized image here . Some of the limiting factors for the Channel Tunnel are: Railways don't like steep gradients The tunnels comprising the Channel Tunnel were excavated using tunnel boring machines (TBMs). Like railways, they cannot tolerate steep gradients. The tunnel was excavated in chalk marl (green coloured material in the picture). This was due to its depth (not being too shallow and not being too deep) and its ability to be easily dug but also it would cause major support issues for...

The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...

If we simplify the whole bridge into 2D thin beam with a constant section size, no internal damping and subject only to small vertical deflections, then the natural frequency is determined by simple harmonic motion: $$ n_0 = \frac{1}{2 \pi} \sqrt{ \frac{ k } { m } } $$ Where $ n_0 $ is the natural frequency, $ k $ is the ratio between restorative force and deflection (the equivalent 'spring stiffness') and $m$ is the mass per unit length of the beam. In a beam the restorative force is the internal shear caused by the deflected shape. As the...


Reading numerous news articles about COVID-19, I got confused as to what is its main transmission route. In particular, I am confused as to whether it is direct landing of respiratory droplets in mouths and noses, contaminated surfaces, aerosols (i.e., suspensions of the virus in the air rather than respiratory droplets quickly landing in mouths or noses or on surfaces), or something else. On the one hand, the website of the Centers for Disease Control and Prevention (CDC) says that contaminated surfaces are not thought to be the main transmission route: It may be possible that a person can get COVID-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose, or possibly their eyes, but this is not thought to be the main way the virus spreads. ( Link ) The same website appears to suggest that the main transmission route is direct landing of respiratory droplets in mouths and noses: The virus is thought to spread mainly from person-to-person. Between people who are in close contact with one another (within about 6 feet). Through respiratory droplets produced when an infected person coughs or sneezes. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. On the other hand, the World Health Organization (WHO), like many other agencies, puts the main emphasis on washing hands. Here is the WHO's first and foremost piece of advice on how to prevent getting infected with the virus: Regularly and thoroughly clean your hands with an alcohol-based hand rub or wash them with soap and water. Why? Washing your hands with soap and water or using alcohol-based hand rub kills viruses that may be on your hands. ( Link ) Social distancing is mentioned on that webpage only as a second piece of advice, and, furthermore, the webpage implies that it is safe to approach coughing people as close as 1 meter away: Maintain at least 1 metre (3 feet) distance between yourself and anyone who is coughing or sneezing. Furthermore, I found an article saying that the virus wasn't detected in the air of hospital rooms with COVID-19 patients, but was detected on surfaces: When researchers in Singapore tested the air in the rooms of three Covid-19 patients, they found no virus particles on cleaned surfaces or in the air even when they took samples on days the patients were symptomatic and presumably shedding virus into the air, they reported this month in the Journal of the American Medical Association. In the room of the third patient, who shed more virus, virus particles were present on ventilation fans and numerous surfaces — but all air samples were negative. ( Link ) But the same article also says that Chinese researchers found COVID-19 aerosols near patients' toilets. My question: Actually what is the main way COVID-19 spreads?

covid-19 infection virus epidemiology coronavirus

Ear tubes do many things to help ear infections. The most important things they do are draining the ear of fluid and ventilating the ear. Overall, this will help your child hear better. The tubes also stop fluid from building up behind the ear drum, which should help prevent the ear infections. Stuff to watch out for There is always the possibility of ear infections coming back after the 6-12 months that the tubes will be in. There is also a minor chance of hearing loss because of scarring of the ear drum. The tube can become blocked, stopping it...

Assuming the person is not taking anticoagulants, it's actually quite difficult to bleed to death from dismemberment of small members (hands, feet, penis, ears, nose, etc). Even large members such as arms and legs are often survivable because the body is very good at protecting itself from blood loss. For example, transected arteries will spasm and clamp off blood flow, and loss of blood will cause the body to divert blood flow away from the extremities and to the vital organs, thereby slowing the bleeding and allowing it to clot. In fact, the whole shock process can be viewed as...

The confusion exists because there are conflicting pronouncements from various authoritative sources but also conflicting pronouncements from the same authority. Covid-19 is a respiratory infection spread by droplets that can be aerosolized (nuclear droplets) in certain situations such as by flushing toilets or in certain medical procedures such as intubation. However, it reaches the air it contaminates surfaces and this allows for the infection to be caught through the eyes nose and mouth with the virus being transferred from the hand. The data from Singapore and Taiwan show that handwashing and social distancing are successful in preventing disease transmission. However,...

Let's get a few things out of the way... Sex is normal ( 1 ). Sexual desire and arousal are normal ( 2 ). Asexuality is normal ( 3 ). Masturbation is normal ( 4 ),( 5 ). There is no problem whatsoever with these three things, although everyone blushes when they talk about them. Is Masturbation healthy? Downsides : Masturbation is not risk free. It is about as dangerous as chewing or walking ( 6 ). Some side effects are irritated skin and ruptured penis, but only if you really, really, really overdo it (Once or twice a day...


Do you bleed to death after your penis is cut off?

I wonder If a person without knowledge and tools cuts off somebody's penis and then doesn't call an ambulance immediately, is there a chance a person may not bleed to death within a short time? I would also like to know if you can still urinate, while the cutting is done without some tool you put into urethra? I haven't found much related to this topic.

Assuming the person is not taking anticoagulants, it's actually quite difficult to bleed to death from dismemberment of small members (hands, feet, penis, ears, nose, etc). Even large members such as arms and legs are often survivable because the body is very good at protecting itself from blood loss. For example, transected arteries will spasm and clamp off blood flow, and loss of blood will cause the body to divert blood flow away from the extremities and to the vital organs, thereby slowing the bleeding and allowing it to clot. In fact, the whole shock process can be viewed as...

Great question! I think it's answerable as an overview, but please know this is only the tip of the iceberg.* Summary : Yes, we have deficits of certain blood products in certain locations at certain times that affect patient care. However, a small percentage of blood product does expire unused (because it wasn't the right product [see background] in the right place at the right time). A little background Donated blood is not usually transfused into a patient as whole blood. Instead, it's broken down into several components which are transfused in different clinical scenarios. The issues of storage and...

Drug molecules, even when stored safely inside a tablet\capsule, inside a closed container, upon a high shelf (hopefully), are still exposed to the environment, and are thus exposed to all of the chemical processes and reactions that go about all around us (to name just a few - oxidation, hydrolysis, isomerization, polymerization, and more). Depending on the type of drug molecule and its functional groups, the molecule may undergo all kinds of chemical processes that may change its structure and\or properties. Therefore, drugs (and foodstuffs, as mentioned in the comment above) degrade and decompose over time, and are thus given...

I'm guessing the "iron" test was actually a hemoglobin test (though I'm certainly no expert in blood donation screens...). I suspect your nurse is reporting a folk tale rather than any true difference. Unfortunately, medical professionals are not always an excellent source of scientific knowledge. I looked for papers that have actually compared measurements in the two hands. Here's one: Patel, A. J., Wesley, R., Leitman, S. F., & Bryant, B. J. (2013). Capillary versus venous haemoglobin determination in the assessment of healthy blood donors. Vox sanguinis, 104(4), 317–323. https://doi.org/10.1111/vox.12006 Capillary fingerstick samples were assayed by HemoCue in 150 donors....


What are left justified and right justified ADC results?

The TI MSP430F20XX series has a 12-bit internal ADC output, which is right-justified. What is the difference between a left-justified output and a right-justified output? What are their pros and cons?

If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few thousand rpm (Revolution Per Minute), so there are dynamic considerations here. Fatigue When materials are subjected to cyclic loading they exhibit a reduction in the allowable stresses. See below for an example: In general, BCC (body-centered cubic) materials (like steel) show a marked drop in strength (close to 50% or more depending on the steel). For these material there is stress (called the endurance limit), for which...

On this processor, the register that holds the conversion result is 16 bits wide. A right-justified result means that bits [( N -1):0] (where N is the number of bits of precision) of the register contain the ADC value and the most-significant bits of the register are set to zero. A left-justified result means that bits [15:(16- N )] of the register hold the result, and bits [(15- N ):0] are set to zero. For example, if your actual conversion result is 0x123, it would be read as 0x0123 if the register was right-justified, and as 0x1230 if it was...

At the beginning of my career (1979) as a design engineer at HP, the mechanical engineer created the part design and then rendered it in pencil on paper, and then transferred it onto sheets of vellum paper held onto a huge flat tilting table called a drafting board which had a precision sliding arm on it with which parallel and right angle lines could be drawn anywhere on the sheet. Clever graphical rules were applied to the resulting drawings which permitted auxiliary views of the part to be generated so it could be viewed from any desired angle, as an...

The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline. They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam. The following quote is from Pipeline Design . It's near the end of the page under Pipe Expansion and Supports Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes...


How do I deal with a compromised server?

I suspect that one or more of my servers is compromised by a hacker, virus, or other mechanism: What are my first steps? When I arrive on site should I disconnect the server, preserve "evidence", are there other initial considerations? How do I go about getting services back online? How do I prevent the same thing from happening immediately again? Are there best practices or methodologies for learning from this incident? If I wanted to put a Incident Response Plan together, where would I start? Should this be part of my Disaster Recovery or Business Continuity Planning? This is meant to be a canonical post for this topic. Originally from serverfault .

Originally from serverfault. Thanks to Robert Moir (RobM) It's hard to give specific advice from what you've posted here but I do have some generic advice based on a post I wrote ages ago back when I could still be bothered to blog. Don't Panic First things first, there are no "quick fixes" other than restoring your system from a backup taken prior to the intrusion, and this has at least two problems. It's difficult to pinpoint when the intrusion happened. It doesn't help you close the "hole" that allowed them to break in last time, nor deal with the...

The reason password expiration policies exist, is to mitigate the problems that would occur if an attacker acquired the password hashes of your system and were to break them. These policies also help minimize some of the risk associated with losing older backups to an attacker. For example, if an attacker were to break in and acquire your shadow password file, they could then start brute forcing the passwords without further accessing the system. Once they know your password, they can access the system and install whatever back doors they want unless you happen to have changed your password in...

An IP address can be set up in DNS to resolve to any host name, by whoever is in control of that IP address. For example, if I am in control of the netblock 203.0.113.128/28, then I can set up 203.0.113.130 to reverse-resolve to presidential-desktop.oval-office.whitehouse.gov . I don't need control of whitehouse.gov to do this, though it can help in some situations (particularly, with any software that checks to make sure reverse and forward resolution matches ). That wouldn't mean that the president of the United States logged into your VPS. If someone has access to your system, they can...

Facebook is allowing you to make a handful of mistakes to ease the login process. A Facebook engineer explained the process at a conference . The gist of it is that Facebook will try various permutations of the input you submitted and see if they match the hash they have in their database. For example, if your password is "myRealPassword!" but you submit "MYrEALpASSWORD!" (capslock on, shift inverting capslock). The submitted password obviously doesn't match what they have stored in their database. Rather than reject you flat out, Facebook tries to up the user experience by trying to "correct" a...


In my opinion, arguments we have been using for years to say that public Wi-Fi access points are insecure are no longer valid, and so are the recommended remedies (e.g. use VPN). Nowadays, most sites use HTTPS and set HSTS headers, so the odds that someone can eavesdrop someone else's connection is very low, to the point of expecting a zero-day vulnerability in TLS. So, what other threats may someone face nowadays on a public network?

Diffie-Hellman is a way of generating a shared secret between two people in such a way that the secret can't be seen by observing the communication. That's an important distinction: You're not sharing information during the key exchange, you're creating a key together. This is particularly useful because you can use this technique to create an encryption key with someone, and then start encrypting your traffic with that key. And even if the traffic is recorded and later analyzed, there's absolutely no way to figure out what the key was, even though the exchanges that created it may have been...

Public WiFi is still insecure, and it will always be if not used together with something like a VPN. Many websites use HTTPS, but not nearly all. In fact, more than 30 percent don't . Only around 5 percent of websites use HSTS. And it's still trust on first use. If the max age is short, that first use can be quite often. Let's face it, even if you are a security pro chances are that you would fall for SSL strip anyway. I know I would. Just because you use HTTPS doesn't mean you do it right. There's still...

Is this normal for a pentest? Absolutely not . Best case scenario: they are performing "social engineering" penetration testing and want to see if you can be pressured into fulfilling a very dangerous action. Middle-case scenario, they don't know how to do their job. Worst-case scenario they are only pretending to be an auditing company and fulfilling their request will result in an expensive breach. In the case of a code-audit the company will obviously need access to source code. However I would expect a company who provides such services to already understand the sensitivity of such a need and...

No, Docker containers are not more secure than a VM. Quoting Daniel Shapira : In 2017 alone, 434 linux kernel exploits were found , and as you have seen in this post, kernel exploits can be devastating for containerized environments. This is because containers share the same kernel as the host, thus trusting the built-in protection mechanisms alone isn’t sufficient. 1. Kernel exploits from a container If someone exploits a kernel bug inside a container, they exploited it on the host OS. If this exploit allows for code execution, it will be executed on the host OS, not inside the...


My email-provider's website ( http://www.gmx.de ) recently started linking to the (German) site http://www.browsersicherheit.info/ which basically claims that due to its capabilities to modify a site's appearance, Adblock Plus (and others) might actually be abused for phising. Here's a quote from that site plus its translation: Solche Add-ons haben Zugriff auf alle Ihre Eingaben im Browser und können diese auch an Dritte weitergeben – auch Ihr Bank-Passwort. Dies kann auf allen Web-Seiten passieren. Sicherheitsmechanismen wie SSL können das nicht verhindern. translated: Such addons can access all your browser's input and can also forward them to third parties - even your banking password. This can happen on all websites. Security mechanisms such as SSL cannot avoid that. Ok, they mention other (pretty obviously crapware) addons, but is Adblock Plus really a security threat or do that site's operators simply use the opportunity to try and scare inexperienced users into viewing their ads again?

web-browser phishing browser-extensions threats

I was one of the implementers of JScript and on the ECMA committee in the mid to late 1990s, so I can provide some historical perspective here. The JavaScript Math.random() function is designed to return a floating point value between 0 and 1. It is widely known (or at least should be) that the output is not cryptographically secure First off: the design of many RNG APIs is horrible . The fact that the .NET Random class can trivially be misused in multiple ways to produce long sequences of the same number is awful. An API where the natural way...

It is not. This is a FUD ( fear, uncertainty, and doubt ) campaign by GMX because they want to display their ads. There is absolutely no security risk from the mentioned ad blockers. They added some crapware to the list to make it look more legitimate. Of course such campaigns are very unusual, especially from such a big and well known company like GMX. Unfortunately, I have no English source at hand (because it's a German only campaign) but since you speak German you may want to read this article at heise.de . Update #1: United Internet, the company...

Here is a dramatization of how the communication goes, when a mail is received anywhere. Context: an e-mail server, alone in a bay, somewhere in Moscow. The server just sits there idly, with an expression of expectancy. Server: Ah, long are the days of my servitude, That shall be spent in ever solitude, 'Ere comes hailing from the outer rings The swift bearer of external tidings. A connection is opened. Server: An incoming client ! Perchance a mail To my guardianship shall be entrusted That I may convey as the fairest steed And to the recipient bring the full tale....

General SSL (and its successor, TLS ) is a protocol that operates directly on top of TCP (although there are also implementations for datagram based protocols such as UDP). This way, protocols on higher layers (such as HTTP) can be left unchanged while still providing a secure connection. Underneath the SSL layer, HTTP is identical to HTTPS. When using SSL/TLS correctly, all an attacker can see on the cable is Which IP and port you are connected to Roughly how much data you are sending What encryption and compression are used In SSLv3 through TLS version 1.2 : the current...


At what point can you walk out of a restaurant if they make you wait to pay the bill?

The other day, I ate at a restaurant; the food was fine, and I got to paying the bill. Their payment system went down. They couldn't accept cards; only cash, and I didn't have the cash on me for the bill. In the end, their payments system was down for about 45 minutes, with several diners being forced to wait around. The staff didn't at any time offer a reassurance that if things weren't fixed in a certain period of time, we could just leave. But it got me thinking to the legality of things - at what point can you just leave? Is it always technically illegal in the UK to leave without paying the bill? Could the restaurant just force you to wait until close of business if necessary? What if they still hadn't fixed the payment system by then?

Technically anyone can sit on a jury. Lawyers are not automatically excluded from juries anymore, as being called for jury duty is a right and a duty that the law abhors automatically excluding people from. That is the official line on this. However, in reality, lawyers will always be stricken from serving by one of the lawyers trying the case. Each lawyer has a certain number of preemptory strikes (the ability to get rid of a juror for any reason, aside from those protected by law, such as race, religion, etc). They also have unlimited "for cause strikes," which are...

There isn't any indication in that news story that the disabled son was anywhere nearby. I agree the situation you describe sounds like a legitimate use of the placard, but it seems in this situation, the placard was being used in a manner totally unrelated to the transport of a disabled person. My guess is that the cops cited her because the son wasn't in the car, and was not inside the establishment at which she parked. California code has this to say: A person to whom a disabled person placard has been issued may permit another person to use...

In the UK Armed Forces, conscientious objection is grounds for a refusal at the admission stage and has been since the end of conscription in 1963. Where a person develops an objection to military service during their term of service, they have the option to appeal for a discharge. Interestingly there's no primary legislation to manage this process but there are established military procedures to take care of this when it happens. Mostly the process seems to be managed informally , with the objector simply being shuffled into a non-combatant role within their regiment and just left there for the...


Why does the minimum variance portfolio provide good returns?

I've been a researching minimum variance portfolios (from this link ) and find that by building MVPs adding constraints on portfolio weights and a few other tweaks to the methods outlined I get generally positive returns over a six-month to one year time scale. I am looking to build some portfolios that are low risk, but have good long term (yearly) expected returns. MVP (as in minimum variance NOT mean variance) seems promising from backtests but I don't have a good intuition for why this works. I understand the optimization procedure is primarily looking to optimize for reducing variance, and I see that this works in the backtest (very low standard deviation of returns). What I don't have an intuitive feel for is why optimizing variance alone (with no regards to optimizing returns, i.e. no mean in the optimization as in traditional mean-variance optimization) gives generally positive returns. Any explanations?

portfolio-management optimization modern-portfolio-theory covariance

Short of having a 'reasonable' predictive model for expected returns and the covariance matrix, there are a couple lines of attack. Shrinkage estimators (via Bayesian inference or Stein-class of estimators) Robust portfolio optimization Michaud's Resampled Efficient Frontier Imposing norm constraints on portfolio weights Naively, shrinkage methods 'shrink' (of course,no?) your estimates (arrived at using historical data), toward some global mean or some target. Within the mean-variance framework, you can use the shrinkage estimators, for both, the expected returns vector, as well as the covariance matrix. Jorion introduced application of a 'Bayes-Stein estimator' to portfolio analysis. Bradley & Efron have a...

There are some cases where you can blend your portfolios using weights directly. One case involves corner portfolios . In this case a linear combination of weights is also efficient. Another case is where you can treat the two separate weights you have produced each as distinct portfolio under the assumption that the correlation between these portfolios is relatively stable. In this scenario, the problem reduces to a two-asset portfolio optimization problem (each asset is simply the linear combination of weights produced via your two methods). The other class of methods involves blending via the expected returns. If you arrived...

The minimum variance solution loads up on securities that have low variances and co-variances. Theoretically you are correct that this should have a low expected return profile. However, it turns out - in contradiction to modern portfolio theory - that securities that have low-volatility or low-beta experience higher returns than high-volatility or high-beta stocks. This is well-documented in the literature as the low-volatility anomaly . As a result, many funds and ETFs have been launched in recent years to exploit this phenomenon. There are a couple arguments as to why the anomaly exists. The paper I cite above argues that...

In general there are two basic ways to make money out of your option pricing models: Sell side (market maker, risk neutral): You use these models to calculate your greeks to hedge your portfolio, so that you live on the spread. Buy side (market/risk taker): You use your model to find mispriced options in the market and buy/sell accordingly. (A third possibility would be to write fancy books and papers about these models and get rich and/or tenure this way ;-)


What is the legal take on the trolley problem?

From the Wikipedia page on the trolley problem: There is a runaway trolley barreling down the railway tracks. Ahead, on the tracks, there are five people tied up and unable to move. The trolley is headed straight for them. You are standing some distance off in the train yard, next to a lever. If you pull this lever, the trolley will switch to a different set of tracks. However, you notice that there is one person on the side track. You have two options: (1) Do nothing, and the trolley kills the five people on the main track. (2) Pull the lever, diverting the trolley onto the side track where it will kill one person. What are the legal aspects of finding yourself in a train yard in the United States when this happens? Is it a crime to pull the lever? Is it a crime to not pull the lever? Does it depend on if you're professionally qualified to pull the lever?

Short Answer Applicants are required by military regulation to have a percentile score on a standardized test called the ASVAB that is 31 or more, which is roughly comparable to an IQ score on the Stanford-Binet scale of a little bit less than 92, for high school graduates seeking to enter the Army or Navy (other services have more strict requirements and applicants with only a GED or to the national guard must have an ASVAB score in the 50th percentile which is equivalent to an IQ of 100). But, U.S. law allows the Department of Defense to allow people...

Parliamentary Supremacy was established by the Glorious Revolution of 1688 in which James II & VII was deposed by Parliament, and the line of succession was changed by Act of Parliament to favor William and Mary. Key laws passed during the aftermath of the Revolution included the Declaration of Right (which forbade keeping a standing army without Parliamentary consent, and put control of the military in Parliament), and the Coronation Oath Act 1688 which established in law obligations of the monarch. Since 1688 it has remained the governing principle of English (later British and UK) law that ultimate authority lies...

Doing nothing is legally safer than doing something, but you're not without hope if you pull the lever. Although you'll likely have committed murder or at least manslaughter, case law is littered with lenience in exigent circumstances, even where convictions have been affirmed. Because this is a philosophical problem, there are plenty of opinions from that perspective, but not so many from a legal standpoint. Let's assume that you're an innocent bystander, (not an employee of the railway company or the train company, etc) and have no duty to act. If you do nothing, then it is unlikely that you...

The answer from @user6726 is a good one. But, I'd like to add to it by pointing out that the body of law applicable to an individual is usually much, much smaller than the entire body of law. I'm a lawyer who has been in private practice for almost 25 years with an extremely diverse practice compared to the average lawyer, and I've never even looked at perhaps 80% of the laws on the books in the states where I practice, and even less elsewhere. By statutory and regulatory volume, the vast majority of statutory and regulatory law is applicable...


What is a Spiral Curve, and How is it Different from a Normal Curve?

I've heard the term spiral curve used to describe a section of highway that is more aesthetically pleasing to the driver's eye. However, I believe I've driven on enough road to say that I can't definitively tell the difference between any given curve other than how "sharp" it is. Could anyone explain how one can determine if a section of curve on a highway is classified as a "spiral" curve, and are there other advantages besides "making it look good"?

civil-engineering highway-engineering surveying rail

As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point. The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in: the edge, the angle would be $\arctan\left(\frac{0.15}{15}\right)=0.57^\circ$ or 1%. the middle ,...

Spiral A spiral curve is a geometric feature that can be added on to a regular circular curve. The spiral provides a gradual transition from moving in a straight line to moving in a curve around a point (or vise-verse). The use of a spiral is about making the road or track follow the same form that the vehicle naturally takes. In a car, you don't go directly from going straight to fully turning. There is a transition area where you slowly turn the steering wheel. Lateral acceleration is slowly increased as the spiral is entered, or it is slowly...


Do I need to worry about galvanic corrosion when inserting a steel bar through aluminium?

My design uses a round steel bar inserted through a hole in an aluminium component with a tight fit (no movement). The operating environment is dry, 20-40°C. Do I need to worry about galvanic corrosion, and if so how long will this last before starting to corrode? Also, what storage conditions should I avoid that would accelerate the corrosion?

Short answer : make it thicker. Long answer : The moment of inertia affects the beam's ability to resist flexing. Use one of the many, free, online moment of inertia calculators (like this one ) to see how increasing the height of the beam will have an exponential effect on increasing the stiffness of the beam. And this site helps provide a pictorial view of the load(s) upon a beam depending upon differing configurations, such as where the supports are and where the load is applied. It also provides a calculator to determine the forces involved. Wikipedia has a decent...

BS5950-1:2000 Clause 1.3.23 defines an H-section as having " an overall depth not greater than 1.2 times its overall width ", and Clause 1.3.25 defines an I section as having " an overall depth greater than 1.2 times its overall width ". Note that at exactly a ratio of 1.2, it would be an H section not an I section.

Very probably, no. Galvanic corrosion required the existence of a reactive and conductive medium, which you don't have. If there is some (for example, at least moist air, or similar) best you can do to paint both of the metals, if you can. Generally, if none of the materials would corrode in an environment, galvanic corrosion is a non-issue (in that environment).

We use engineering strain even though it is not the "correct" value because in most cases, specifically in the elastic regime, engineering strain differs negligibly from true strain. For linear elastic, Hookean materials, it is generally the case strain at the elastic limit is very small. Even the strongest steels, for example, have an upper limit when cold worked of about $\sigma_{\textrm{el}}=1\times 10^{9}\ \textrm{Pa}$. The modulus of steel is approximately $E=200\times 10^{9}\ \textrm{Pa}$. Thus $\varepsilon_{\textrm{el}}=0.005=0.5\%$ for the strongest steels. So at the onset of plastic deformation, engineering strain is $0.5\%$. Many useful elastic materials have much lower engineering strain at...


Vitamin B12 is stored over a very long time such that vegans, for example, only get any signs or symptoms of B12 deficiency after a span of years, even though they have hardly any sources of the vitamin in their diets. The dose required daily to prevent disease seems to vary dramatically, or maybe it is very hard to determine accurately. For example: This site says pregnant women need less than 3 micrograms (µg)/day (where pregnant women need more than non-pregnant women). A friend of mine got her blood values in order by ingesting 7 µg/day. My apothecary person told me 500-1000 µg per day are needed to even start absorbing meaningful quantities by ingestion. My doctor prescribed 200 µg/day for me. Another factor is that apparently your intestinal bacteria determine the rate of absorption. So what is the amount of B12 needed per day? Please note that this is not about deficient absorption, where the amount would be above what healthy people need.

obstetrics b-12-supplements absorption-absorb apothecary prenatal-vitamins

The estimated daily requirement needed to maintain body stores of B12 varies, an estimate being from 2µg to 5µg, more if stores have been depleted in any way. It is estimated that the average person stores about 1 mg (1000 µg) of B12 in their liver, and other smaller amounts elsewhere. The recommended daily allowance assumes a 50% absorption rate of ingested B12. This is a good question to discuss the limits of medical science. How is the requirement for B12 determined? Longitudinal studies are those that follow people over many years (even decades.) Humans are not subjected to longitudinal...

The short version is that in 2016 the polio vaccine changed. A more thorough explanation requires some background on the immunology of polio and its vaccines, which is not straightforward. Polio virus is usually harmless, it reproduces in the gut and spreads through a fecal-oral route. In ~99% of infections it only causes mild diarrhea. In the remaining 1% of cases, however, it gets into the bloodstream and from there enters nerves, causing paralysis and/or respiratory failure. There are two types of vaccine: inactivated polio vaccine (IPV) and oral polio vaccine (OPV). IPV is a shot of killed virus particles...

In general, the benefit of flu shots is to the general population. Taking the cited value of 40%-60% from the CDC, we can say that it might be a coin toss for you personally to be protected from influenza by the shot. However, if many thousands of people take the flu shot, even if it only works on 40% of them, it helps protect the remaining population from the spread of the virus (AKA herd immunity). This is especially important to those within the population who cannot access the shot or cannot receive it due to a preexisting medical condition....

The seasonal coronaviruses attach exclusively to cells with a ciliated epithelium. Coronaviruses invade the respiratory tract via the nose. After an incubation period of about 3 days, they cause the symptoms of a common cold, including nasal obstruction, sneezing, runny nose, and occasionally cough (Figs. 60-1 and 60-2). The disease resolves in a few days, during which virus is shed in nasal secretions. There is some evidence that the respiratory coronaviruses can cause disease of the lower airways but it is unlikely that this is due to direct invasion. Other manifestations of disease such as multiple sclerosis have been attributed...


At what point is uploading a movie with heavy compression not copyright infringement?

My friend recently linked me a GIF of the entire Shrek movie in 60x60 resolution . At the time of writing this, it has 12 million views on imgur. This got me thinking, could a copyright holder technically sue over the uploading of this GIF? I'm guessing yes, because it's still barely legible. But at what point are they not allowed to do so? Surely they can't claim intellectual property over one pixel shifting to specific colors for 90 minutes, for example? How about 2x2? Etc. I read What considerations determine copyright infringement? which to me makes it seem like they COULD sue over the one pixel shifting colors for 90 minutes, because I did copy and modify the movie. Is that really true? I also read How similar should a tune be to constitute theft of someone else's tune? which states that it depends a lot on the case since there are so many variables. Is that also true in this case? I imagine songs work very different from movies. Looking for both US or EU answers.

united-states copyright intellectual-property internet european-union

Through the legal doctrine of "transferred intent", wherein if one intends to murder A, and undertake actions to kill A, but one's actions kill B, one has murdered B. Whatever crimes one would have committed, had one performed them on one's intended target, are considered committed against the individual one actually performed them on. Many crimes require one to have mens rea to be guilty; they do not require one to have mens rea towards a given individual. So, so long as one had the proper intent to murder someone, the actual victim of their actions is irrelevant.

Maybe, Hence the Lawsuits In the absence of clear statute law these all circle around tort law. For the scooter companies, trespass to chattels , and for the affected landowners (who hire the removalists) trespass to land and nuisance seem applicable. In essence, I can’t take your stuff (trespass to chattels) but you can’t leave your stuff on my property (trespass to land) or impeding access to it (nuisance). If you do, I am entitled to the reasonable costs of dealing with it. Note that, as owner, you remain responsible for you stuff even if you rented it to someone...

In the case of United States vs Wong Kim Ark 169 U.S. 649 (1898) (a 6-2 decision), the Supreme Court wrote: [T]he real object of the Fourteenth Amendment of the Constitution, in qualifying the words, "All persons born in the United States" by the addition "and subject to the jurisdiction thereof," would appear to have been to exclude, by the fewest and fittest words (besides children of members of the Indian tribes, standing in a peculiar relation to the National Government, unknown to the common law), the two classes of cases -- children born of alien enemies in hostile occupation...

A few years ago, there was a trial in the USA about some short sound on some music CD: One party claimed that one piece of music on the CD contained a sound of less than one second length which is copied from another CD without the permission of the copyright owner of the other CD. It could never be found out if this claim was really true. The court's decision was: If it is not possible to distinguish between a copy and a work that does not depend on the other work at all, it is not a copyright...


Search for military installed backdoors on laptop

My laptop was confiscated by the military institute of my country and they made me to give them all my passwords (I cannot tell you the name of my country). They did not give it back to me for one week (yes, it was out of my sight for a while). I nuked it from orbit but I just realised that it was on sleep state for 2 days and not in shutdown state, so it was connected to my modem via wifi. Does it need to be worried about? and I need to make sure if they have added something to monitor my activities or steal my data or not? And if they have done that, what should I do to prevent them. I have double checked the laptop physically and there is no sign of screw or plastic deformation. Is that still possible that they have compromised its hardware?

Facebook is allowing you to make a handful of mistakes to ease the login process. A Facebook engineer explained the process at a conference . The gist of it is that Facebook will try various permutations of the input you submitted and see if they match the hash they have in their database. For example, if your password is "myRealPassword!" but you submit "MYrEALpASSWORD!" (capslock on, shift inverting capslock). The submitted password obviously doesn't match what they have stored in their database. Rather than reject you flat out, Facebook tries to up the user experience by trying to "correct" a...

If the device left your sight for any amount of time, replace it. It can no longer be trusted. The cost to assure it can still be trusted significantly exceeds the cost of getting a new one There is effectively no way to verify that the hardware has not been tampered with without significant expertise and employing non-trivial resources. The only solution is to replace the laptop and all associated components. Without knowing your country or other aspects of the situation you are in, there is no way for me to comment on the likelihood of this, only on the...

I was one of the implementers of JScript and on the ECMA committee in the mid to late 1990s, so I can provide some historical perspective here. The JavaScript Math.random() function is designed to return a floating point value between 0 and 1. It is widely known (or at least should be) that the output is not cryptographically secure First off: the design of many RNG APIs is horrible . The fact that the .NET Random class can trivially be misused in multiple ways to produce long sequences of the same number is awful. An API where the natural way...

I think the safest option for you would be to use Qubes OS with its built in DisposableVM s functionality, and its “ Convert to Trusted PDF ” tool. What is Qubes OS? Qubes is an operating system where it's all based on virtual machines. You can think of it as if you had different isolated ‘computers’ inside yours. So that way you can compartmentalize your digital life into different domains, so that you can have a ‘computer’ where you only do work related stuff, another ‘computer’ that is offline and where you store your password database and your PGP...


Help! My home PC has been infected by a virus! What do I do now?

This is an attempt to ask a canonical question as discussed in this old meta post . The goal is to create something helpful that can be used as a duplicate when non experts ask about virus infections. Let's say that I have determined beyond doubt that my home PC is infected by a virus. If necessary, you can assume that my computer runs Windows. Answers aimed at the non-technical reader are encouraged. What do I do now? How do I get rid of the virus? Do I really need to do a full reinstall? Can't I just run a couple of anti-virus programs, delete some registry keys, and call it a day? I really don't have time to deal with this right now. Is it dangerous to keep using the computer while it is infected? I don't have backups of my family photos or my master thesis from before the infection occurred. Is it safe to restore backups made after the infection occurred? Do I need to worry about peripherals getting infected? Do I need to do anything about my router or other devices on my home network?

malware virus antivirus antimalware virus-removal

General SSL (and its successor, TLS ) is a protocol that operates directly on top of TCP (although there are also implementations for datagram based protocols such as UDP). This way, protocols on higher layers (such as HTTP) can be left unchanged while still providing a secure connection. Underneath the SSL layer, HTTP is identical to HTTPS. When using SSL/TLS correctly, all an attacker can see on the cable is Which IP and port you are connected to Roughly how much data you are sending What encryption and compression are used In SSLv3 through TLS version 1.2 : the current...

Take a look at this USB keyboard : "But that's not a keyboard! That's an USB drive, silly!" Actually, no. It looks like a USB drive to you, but when it gets connected to a computer, it will report that it is a USB keyboard. And the moment it is installed, it will start typing key sequences you programmed on it beforehand. Any operating system I know automatically trusts USB keyboards and installs them as trusted input devices without requiring any user interaction the moment they are connected. There are various payloads available for it. For example, there is one...

What do I do now? How do I get rid of the virus? The best option is what is referred to as " nuke it from orbit ." The reference is from Aliens : The idea behind this is that you wipe your hard drive and reinstall your OS. Before you do this, you should make sure you have the following: A way to boot your computer off installation media. This can be in the form of the Install CD that came with your computer, or a DVD you burnt from an ISO file (Windows can be downloaded legally here...

Diffie-Hellman is a way of generating a shared secret between two people in such a way that the secret can't be seen by observing the communication. That's an important distinction: You're not sharing information during the key exchange, you're creating a key together. This is particularly useful because you can use this technique to create an encryption key with someone, and then start encrypting your traffic with that key. And even if the traffic is recorded and later analyzed, there's absolutely no way to figure out what the key was, even though the exchanges that created it may have been...