h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: Manhole Covers - A7822442
Author: CyberLama - U2730198

I know there were several comments on this regarding the details of sewage systems but this topic is just about manhole covers.

I would be more than happy to write up several more entries that cover manholes, sewage lines and history to cover that aspect.

Comments if you will.

It looks good

Just a few small things...

"Because of weight, they are easier to line up than triangle and square covers."
This needs explaining a little clearer. Maybe something like, it is easier to line up a circular object to its hole as it is symetrical in all directions. This is useful the covers are heavy so repositioning would be hard work.

"Manhole covers are made of iron or concrete for larger systems where weight is a concern."
I dont really understand this. Weight is a concern in what way? You already stated that the covers are usually made of iron.

"Some are also smooth while others have a pattern to assist traction of vehicles passing over them."
I dont think the 'also' is needed

"Since manhole covers tend to weigh more than 50kg, manhole cover pictures are sometimes collected as works of art."
Why would the fact that a manhole cover is heavy make it more likely to be art?

And finally, it would be nice to add some of the links i suggested before.

Good work,

SI x

I think you've misse ths most important reason for a manhole being round. Any other shape could be turned on it's side and fall down the hole.

I hasten to add that I was told this, and haven't tested it, so I'm not entirely sure it's true...

Actually there is one other shape that will not fall through. I believe I did mention the fact that they are round and cannot fall through.

What is the other shape that will not fall though other than round?

An equilateral triangle won't fall through either.

- Casper.
The friendly hootoo

I thought you said they couldn't fall through because of the lip? Which I thought was a different thing.

Because of weight, they are easier to line up than triangle and square covers.> triangular

It is the lip which stops both circular and equilateral triangular covers falling through. If there were no lip, any shape would drop straight down.

The thing about circular / equi-triangular covers is that it doesn't which way you turn them, they won't drop down as long as there's any lip at all. Contrast that with a square cover (for example) which could be turned on its side and dropped down the diagonal, which is longer than the sides.


Merry Christmas to one and all.

I get it now.

I'd say that the approximatly is a bit to accurate to me approximate

* Unlike movies where a person would fall through a manhole as a joke and survive with minimal injuries, if you were to fall into a manhole unsuspectingly you would be seriously injured or could possibly even be killed from the fall.

> this is only true if the sewege system is good.
In Israel, most manholes are only cover a hole that's up to a couple of feet deep, and there's no mortal risk in falling ito one.



Dancer

Oh, and -
Nice entry


Dancer

I don't understand how an equilateral triangle would be less likely than a square to fall through. The difference between the longest distance across and the shortest is I believe greater. If the shortest distance is 1(the side on a square and the height of the triangle), the longest distance on the square(from any corner to its opposite) will be the square root of 2, and the longest distance on the equilateral triangle(one side) will be 2 divided by square root of 3. 1.14<1.15
So, if the lip added around the cover is such that a square will fall through when you tilt it up and slide it in off-center(a lip of less than .07, roughly), an equilateral triangle cover will be able to slide in along one side of the hole as long as it's oriented a bit off.

You are correct. An equilateral triangle will fall just as readily as a square (which is an equilateral rectangle).
Actually an equilateral anything will fall through, but the more sides it has, the smaller the lip needs to be to prevent it.

a circle is equilateral with infinite sides, and therefore any lip will do.


Dancer

Wrong, I'm afraid.
We're talking about the cover and hole being the same shape, but with the lip inside the hole making it very slightly smaller. (It doesn't matter how much smaller.)

Let's use some examples.

Take a square with sides of 1m. As Black_Carrot correctly stated, the distance across the diagonal of the square will be the square root of 2 (1.414m). So if you lift a square cover to the vertical and turn it through 45 degrees, the 1m side will fit inside the diagonal (with ~40cm to spare). Therefore the cover can drop down the hole.

Now take an equilateral triangle with sides of 1m. The height (altitude) of the triangle is the square root of 1 squared minus 0.5 squared = the square root of 0.75 = 0.86m (i.e. less than 1m). In fact, it is impossible to draw a line anywhere inside the triangle which is longer than 1m. So it's impossible to turn the cover and drop it down the hole.

The greatest internal length in a circle is the diameter, so as long as there's any lip at all (no matter how small) in the hole, it's impossible to drop a circular cover down it.


- Casper (wishing it was possible to include diagrams in posts )

Diagrams aren't necessary, I see what you're saying. The problem is, though, that the side isn't meant to be the smallest length across, the height is. The side is then much longer, so if you tilt an equilateral triangle up until one side is vertical and its matching altitude is horizontal, you could slide it easily in along one side of the hole. This something I can easily see happening by accident on a daily basis, if the cover is ever left near the hole. All it takes is someone nudging it a little farther, and it could tip in. Also, whether the lip is attached to the outside of the shape or carved into it, the proportions remain what they are.

Diagram:
http://www.prot42.org/mhc.jpg


Dancer

Interesting Entry CyberLama

Under Header 'Why Manhole Covers Are Round'

>"Because of weight, they are easier to line up than triangle and square covers."

Should that be because of their shape?

In that section is the first time you mention triangle or square manhole covers, could you mention those before then, are different utilities recognissed by the different shapes?

Emmily

Who'd have thought it was possible to write a whole entry on Manhole Covers? I saw this on PR and was expecting it to be rubbish, so I was very pleasantly surprised when I read it? Well done, CyberLlama!

A few comments:

"Manhole covers are usually circular in shape, made of iron and found on top of manholes. Manholes are entry points, located on the ground, which usually provide access into the underground sewage or utility areas of a city."

I would swap these sentences around. It makes more sense to efine them before you talk about their shape.

"From as early as 3500BC slabs of stone or pieces of wood were used to cover the ditches that carried sewage waste."

Do you know where this was? If so, can you mention it? Also, you don't need to say "sewage" and "waste".

"sewage systems designs that are implemented today were not developed until somewhere between 1700AD and 1800AD."

Perhaps this would be better as "sewage systems designs similar to those used today were not developed until somewhere between 1700AD and 1800AD."

"These sewage lines had a hole dug as the last point from either line in order to connect the two systems together. A cover was then placed over the junction point." Maybe I'm being dumb here, but I don't see how a hole from the surface to the end of a sewer connects it with another sewer.

"This junction served as a plumbing point in case the sewer became clogged with debris and backed up. Sometime in the 19th Century manhole covers started being made of iron." The second sentence seems out of place here. Perhaps you might consider making it a new paragraph. Also, what were the covers made from before?

"1 meter long by 1 meter wide" - house style is British spellings -> "metre"

"An access hatch was constructed and placed over the top of the manhole to prevent the unfortunate situation that someone discovered himself directly above the manhole at which point he would immediately experience the laws of gravity in action." I know what you're trying to say, but it's awkwardly worded. You don't have to be funny in EG entries (I'm usually not, but that's another story. )

"Today these hatches are known as companion ways or hatches." Perhaps -> "Today these openings are known as companion ways or hatches."

"Man hole covers" -> "Manhole covers"

"several popular reasons" What do you mean by "popular reasons"?


"Most iron manhole covers have an artistic or aesthetic print forged on them." Perhaps this woud be a good place to mention that even today, in Rome, manhole covers have the letters "SPQR" on them, which stands for "Senatus Populusque Romanus" meaning "The Senate and the People of Rome". This was used in Roman times (according to Wikipedia *spit*) as the official name for boh the Republic and later the Empire. Today it appears in the coat of arms of Rome and it was Italian dictator Mussolini who ordered that it be placed on the manhole covers.

"The size of a manhole cover varies but measures approximately 60.96cm radius." Surely this would vary depending on the country.

By the way, is a manhole cover different to a drain? Most roads here (in the UK) have square/rectangular drain covers with open slits at the side of the road to allow rain water to drain away. I don't recall having seen a circular manhole cover in the middle of the road. Maybe I've just forgotten, but it seems like the kind of thing you see in films and not in real life. Maybe it's American?

Anyway, great entry!