Man, I cannot believe you guys!
The horsepower unit was defined by James Watt himself. His steam engine
was developed to pump water out of mines, a job previously done by horses.
To get an idea of how much work his machine would have to do to replace
them, he studied how much water the average horse lifted up a shaft of so
many feet during a work shift. Unit conversions and a little rounding
produced the 550 ft-lb/sec figure we all know. (BTW, it's because he looked
at average effort over a full shift that explains why the horsepower unit is
so small. A horse's full effort is many times more than one horsepower,
a conundrum that puzzled me greatly as a child.)
The famous episode of the horses pulling apart hemispheres has nothing
to do with horsepower. That was in Germany, shortly after the invention
of the vacuum pump, and was just a demonstration of the existence (and
magnitude) of air pressure.
Have you been watching Connections by James Burke? He
covered this as well as the vacuum pump bit. If you haven't
its full of great stuff all from a Brit.
Nelson.
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