_Answer._--Before the invention of the engine, horses were employed in
mills and mines, and other places; the number of horses employed in a
mill or mine, indicating the amount of work going on, and the necessity
of employing them, and when the steam engine came on the scene, and a
purchaser wanted, he was told that the engine was equal to so many
horses; that comparison gave the purchaser a clear idea of the engine he
required. Savory was the first to suggest this comparison, but Watt knew
that horses differed in size and strength, and in order to be sure of a
safe standard for his engine power he experimented with big horses in
some London breweries, and after careful calculation and comparison he
fixed a horse-power at 32,000 lb., that is to say, that a horse could
lift that weight of water one foot above the ground in a minute for
eight hours per day. This standard has remained ever since, although it
is above the average of the power of the average horse, it is in favour
of the purchaser of an engine, as well as being capable of working more
than eight hours a day, or twenty hours if required.