Some time ago I wrote about the relationship between thrust and horsepower, and the question of why one is used to describe the output of pure reaction engines and the other that of engines driving propellers. I argued that the reason was historical. The great majority of engines-steam, gasoline or what have you-have been designed to bring about some sort of mechanical rotation. The force driving a rotating shaft, multiplied by the speed of rotation, provides a straightforward way of measuring the rate at which a machine can do work, which we call its horsepower.
How horses got involved in all this is another matter. A human athlete is capable of brief bursts of about one horsepower, and continuous output at about a third of one. I would have thought that a normal horse, being much larger than a human being and having eaten fewer Twinkies, should be capable of putting out much more than one horsepower; but the Oxford English Dictionary avers that horses possess only three-fourths of a horsepower. I invite readers to contemplate this koan-like paradox at their leisure. At any rate, we apparently owe the term horsepower, which came into use around the start of the 19th century, to James Watt. Watt developed the first practical steam engine, and in modern times his name has replaced the horsepower as a measure of engine power wherever the metric system is used, which is to say, almost everywhere in the world except here. The watt is a relatively puny thing, however-without umbrage, I hope, to the excellent Mr. Watt-and it takes about 750 of them to make a single horsepower.
