One would actually think round manhole covers have several disadvantages to offset their one purported strength. For example, it would be more difficult to attach a hinge or locking mechanism. Also, the grip hole would not be guaranteed to be in the same alignment every time - if you were using a machine to raise the cover, you might have to manouevre the machine into incovenient angles to properly access the grip, according to the whim of the previous team who replaced the cover. With a square lid, the alignment could be standardised to the most convenient position for machine access. Also, assuming the bars in the ladder are straight, which AFAIK they always are, placing a ladder in a round hole wastes space.
Feynmann's answer about the cylinder being the strongest structure against compression is true but one wouldn't expect this to be a critical factor at typical street-access depths. Maybe if you were going down 50m or more, but even so, dirt is hardly as effective at exerting pressure as a free-to-move liquid.
For what it's worth, I spent a summer pulling off manhole covers for a living. There's no machine required, just a bent rod with a handle, and they're easy enough to open.
And for the record, yes, it is quite easy to drop the rectangular covers into the hole.
With a square lid, the alignment could be standardised to the most convenient position for machine access.
Small quibble, but most of those manhole covers and such are placed where they are because someone built some kind of tunnel there a hundred years ago (for whatever reason, from sewage to water to anything else). The layout of the streets then often evolves largely independently of the position of the manhole covers, thus resulting in some "interesting" placements of manhole covers, sometimes not all that accessible (though I guess it depends on how strict the local regulators are on the architects who design the new layout).
Ah yes, like so many things it is probably for "legacy reasons". And needless to say really old manhole tunnels would definitely tend to be round, especially if they were constructed out of shaped stones, where the shape of the structure is an important contributor to its integrity.
It is easier to replace a circular cover, since you don't have to worry about it's orientation. I suspect that this is the main reason to use a circular manhole cover over some other shape.
But then it would have to be lifted straight up, which would require a lot of force. Are they really like this? Now you've piqued my curiousity, I'm going to look on the street!
I had a car accident because of a rounded manhole cover. My wife had to emergency stop because a rounded cover in the middle of the road was half open, sitting vertically on top of the hole. Thankfully no one got hurt.
I was asked this once during an interview. After I gave several lengthy possible explanations, the interviewer was basically speechless.
The previous question was much more interesting. I was asked to estimate the number of piano tuners there were in my city. I got it right, and it turns out it was one of the the class Fermi questions
Still, I'm not really a fan of these types of questions. Chances are any curious person who spends a lot of time on the net would have come across them some time or another.
I have seen man hole covers that were in the shape of other curves of constant width!! as in the rotary engine shape he describes. Reminds me of the convex solids class I took at UCLA years ago.
Boy, I always wanted to say that! If only I could pull one of those interviews.
Where did you see those manhole covers? Its seems a bit over-the-top to manufacture a manhole cover in a weird shape when round or rectangular will do.
This reminds me about the story where Feynman is being interviewed by a psychologist. He answers each question in a way that is both perfectly reasonable yet would cause the psychologist to become increasingly concerned.
Thanks. I was looking for a "search in book"-type link but couldn't find it. There are some excerpts on e2 that I found, but not of the passage that I was thinking of.
For what it's worth, I'm interning at Microsoft this summer, and none of my interviewers (6 or so) asked questions like this. Most of the questions had to do with arrays of integers, with one linked list question and 1-2 C string questions.
This isn't trivia. Creating a new thread is expensive; so understanding the difference can make a huge difference in performance.
Saying "I can google this if I need to" is a cop-out: Yes, you can google if if you realize that it matters, but if you don't know the answer you're probably not going to realize that it matters.
I see what you're saying, but in practice I see no correlation between people with great recall of arcane functions and syntax and good developers.
And the question wasn't, "What's the difference between pulling a thread from the pool and spinning up a new one?" That's a pretty good question, if you're going to be doing programming with threads.
I don't have any meaningful statistics on this myself, but I always found this passage from The Jargon File interesting:
Another trait is probably even more important: the ability to mentally absorb, retain, and reference large amounts of ‘meaningless’ detail, trusting to later experience to give it context and meaning. A person of merely average analytical intelligence who has this trait can become an effective hacker, but a creative genius who lacks it will swiftly find himself outdistanced by people who routinely upload the contents of thick reference manuals into their brains.
The cost of someone trying to upload huge quantities of trivia is a failure to update when systems change. Consider the cost of creating and maintaining high quality comments vs readable code.
So I'll assume, since you think that "I can google this" is a cop-out, that you never hit up Google in real life to look up some obscure technical point about a function that you need to call once every couple months in your coding? You know every interface and implementation detail for every standard library call in your language of choice?
I call shenanigans.
In our profession trivia is useful and even extremely important to be able to keep in your head; however, it's all but useless as an interview filter unless it's highly targeted to a topic that the job description requires expert knowledge of.
Feynmann's answer about the cylinder being the strongest structure against compression is true but one wouldn't expect this to be a critical factor at typical street-access depths. Maybe if you were going down 50m or more, but even so, dirt is hardly as effective at exerting pressure as a free-to-move liquid.