Why Are Manhole Covers Round?

iStock
iStock

If you’re in the habit of looking down at your feet as you walk, you may have noticed that most manholes, and their covers, are round. Or perhaps you’ve been in a job interview where your prospective employer asked this question (which apparently happens enough that it was included in the third edition of The Complete Idiot’s Guide to Job Interviews). He wants to know about your ability to think on your feet, and isn’t so much interested in the correct answer, but it is a good question. Of all of the potential shapes for manholes, why was a circle chosen?

There are a number of reasons, according to Larry Scheckel in his book Ask A Science Teacher: 250 Answers to Questions You've Always Had About How Everyday Stuff Really Works. Manholes are round because “it is the best shape to resist the compression of Earth around it.” Round shapes are easier to manufacture than square or rectangular shapes, and because manhole covers are heavy, being round makes them easier to move from place to place (just roll them!). As an added benefit, workers don’t need to line up the covers with any angles, making round covers easy to slip into place.

But perhaps the biggest reason that manhole covers are round is that round covers can’t fall through a circular opening. “For all manholes, there is a ‘lip’ around the rim of the hole, holding up the cover, which means that the underlying hole is smaller than the cover,” Scheckel writes. “A round manhole cover can’t fall through its circular opening, because no matter how you position it, the cover is wider than the hole. But a square, rectangular, or oval manhole cover could fall in if it was inserted diagonally into the hole,” which would be bad news for unobservant pedestrians and drivers alike.

Is There An International Standard Governing Scientific Naming Conventions?

iStock/Grafissimo
iStock/Grafissimo

Jelle Zijlstra:

There are lots of different systems of scientific names with different conventions or rules governing them: chemicals, genes, stars, archeological cultures, and so on. But the one I'm familiar with is the naming system for animals.

The modern naming system for animals derives from the works of the 18th-century Swedish naturalist Carl von Linné (Latinized to Carolus Linnaeus). Linnaeus introduced the system of binominal nomenclature, where animals have names composed of two parts, like Homo sapiens. Linnaeus wrote in Latin and most his names were of Latin origin, although a few were derived from Greek, like Rhinoceros for rhinos, or from other languages, like Sus babyrussa for the babirusa (from Malay).

Other people also started using Linnaeus's system, and a system of rules was developed and eventually codified into what is now called the International Code of Zoological Nomenclature (ICZN). In this case, therefore, there is indeed an international standard governing naming conventions. However, it does not put very strict requirements on the derivation of names: they are merely required to be in the Latin alphabet.

In practice a lot of well-known scientific names are derived from Greek. This is especially true for genus names: Tyrannosaurus, Macropus (kangaroos), Drosophila (fruit flies), Caenorhabditis (nematode worms), Peromyscus (deermice), and so on. Species names are more likely to be derived from Latin (e.g., T. rex, C. elegans, P. maniculatus, but Drosophila melanogaster is Greek again).

One interesting pattern I've noticed in mammals is that even when Linnaeus named the first genus in a group by a Latin name, usually most later names for related genera use Greek roots instead. For example, Linnaeus gave the name Mus to mice, and that is still the genus name for the house mouse, but most related genera use compounds of the Greek-derived root -mys (from μῦς), which also means "mouse." Similarly, bats for Linnaeus were Vespertilio, but there are many more compounds of the Greek root -nycteris (νυκτερίς); pigs are Sus, but compounds usually use Greek -choerus (χοῖρος) or -hys/-hyus (ὗς); weasels are Mustela but compounds usually use -gale or -galea (γαλέη); horses are Equus but compounds use -hippus (ἵππος).

This post originally appeared on Quora. Click here to view.

Can Soap Get Dirty?

iStock/vintagerobot
iStock/vintagerobot

When you see lovely little bars of lemon-thyme or lavender hand soaps on the rim of a sink, you know they are there to make you feel as fresh as a gardenia-scented daisy. We all know washing our hands is important, but, like washcloths and towels, can the bars of hand soap we use to clean ourselves become dirty as well?

Soaps are simply mixtures of sodium or potassium salts derived from fatty acids and alkali solutions during a process called saponification. Each soap molecule is made of a long, non-polar, hydrophobic (repelled by water) hydrocarbon chain (the "tail") capped by a polar, hydrophilic (water-soluble) "salt" head. Because soap molecules have both polar and non-polar properties, they're great emulsifiers, which means they can disperse one liquid into another.

When you wash your dirty hands with soap and water, the tails of the soap molecules are repelled by water and attracted to oils, which attract dirt. The tails cluster together and form structures called micelles, trapping the dirt and oils. The micelles are negatively charged and soluble in water, so they repel each other and remain dispersed in water—and can easily be washed away.

So, yes, soap does indeed get dirty. That's sort of how it gets your hands clean: by latching onto grease, dirt and oil more strongly than your skin does. Of course, when you're using soap, you're washing all those loose, dirt-trapping, dirty soap molecules away, but a bar of soap sitting on the bathroom counter or liquid soap in a bottle can also be contaminated with microorganisms.

This doesn't seem to be much of a problem, though. In the few studies that have been done on the matter, test subjects were given bars of soap laden with E. coli and other bacteria and instructed to wash up. None of the studies found any evidence of bacteria transfer from the soap to the subjects' hands. (It should be noted that two of these studies were conducted by Procter & Gamble and the Dial Corp., though no contradictory evidence has been found.)

Dirty soap can't clean itself, though. A contaminated bar of soap gets cleaned via the same mechanical action that helps clean you up when you wash your hands: good ol' fashioned scrubbing. The friction from rubbing your hands against the soap, as well as the flushing action of running water, removes any harmful microorganisms from both your hands and the soap and sends them down the drain.

This story was updated in 2019.

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