Why are manhole covers round?

Cecil replies:

Why are manhole covers round?

–Barry L., Glendale, Arizona

PS: This is no joke.

Cecil replies:

Hey, man, the Straight Dope is always serious–it’s REALITY that’s a joke. The commonly told story, which may even be true, is that manhole covers are round so some moron from Streets and Sanitation, or whatever it is they have out there in Glendale, won’t be able to drop them accidentally into the manhole.

This may require some thought. You take your squares, for instance. A square cover you can tip up on edge and drop through the hole on the diagonal, if you follow me. Similarly with ovals. A circular hole, however, will defy the most determined efforts in this regard. (Bear in mind that there’s a lip around the inside of the manhole that the cover rests on; this effectively makes the diameter of the manhole an inch or so less than that of the cover.) This means manholes are one of the few things in the universe that are absolutely goofproof.

Kinda reassuring, except for one thing. A circular manhole cover isn’t the only shape that won’t fall into the hole. A cover shaped like an equilateral triangle won’t fall in either. [Not quite. See below.]  The main objections to be raised against a triangular cover are that (1) it would be pretty tough to roll; (2) the corners might tend to pop up when cars drove over them (I get the latter from my friend David Feldman’s book, When Do Fish Sleep?); and (3) it would look stupid. Regarding point #3, someone has written me claiming the English have triangular manhole covers. ‘Nuff said.

Almost forgot. Point #4. Circular covers, in addition to the won’t-fall-in angle, have the advantage that you don’t have to get the corners lined up when dropping them into place, saving God knows how many thousands of seconds of valuable time. Originally I wrote this as “man-seconds,” to which I added the hilarious remark, “OK, worker-seconds, but how many female sewer workers have YOU seen?” Caught no end of flak from numerous defenders of insulted female sewer workers. Guess I won’t try THAT again.

Anyway, it seems to me that round has got to be the shape of choice. That said, we don’t have any definitive statements on this subject from the Manhole Cover Research Institute. We cannot entirely discount the possibility that the covers are round because someone at the dawn of modern civil engineering discovered he could draw them by tracing around the outside of a penny. But good enough for now.

NOT SO FAST

The Teeming Millions have pointed out at exasperating length that it’s not an equilateral triangle that won’t fall into the manhole but a Rouleaux triangle–the kind in which each side consists of an arc centered on the opposite vertex (or at least that’s one way to do it). A puffy triangle, if you will. An equilateral triangle can be dropped into the manhole by positioning it so that one of the altitudes is parallel and close to one side of the hole (you geometry lovers know what I mean). A Rouleaux triangle, however, is a constant-diameter shape, and can’t be. As usual, blame Little Ed. I’m not saying it’s his fault, just that you should blame him.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.