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Re: Watt's definition of horsepower



Not for the first time, I would like to thank Larry Cartwright
(who is undoubtedly an adornment of the Charlotte and the
Michigan school system) for this scholarly essay.

Brian Whatcott

At 00:37 12/7/00 -0500, you wrote:
James Watt conducted a number of studies of horses in practical
situations, from mine ponies to the huge Clydesdale dray horses. His
final assessment came from the most common "industrial" use of horses at
the time, which was the horse gins used for pumping water and hauling
loads of rock debris and coal from mines.

Most commonly a shift was 12 hours.

A horse gin (or horse whim) has a horse harnessed to the end of a long
bar. The other end of the bar is attached to a central capstan, and the
horse walks around and around a circle of about 12 foot radius hour
after hour. Some of the big horse gins used as many as a dozen horses
turning the same capstan, but the one horse gin was most common.

Watt determined that a typical strong horse, worked hard during a shift
on a gin, exerted about 180 lbs of force at a rate of approximately
22,000 ft-lb/minute or 370 ft-lb/second. However, in defining his
Horsepower unit, he increased the power by 50% to 33,000 ft-lb/minute or
550 ft-lb/second.

In 1843 engineer Frederick Walter Simms presented a paper "Upon the
Measurement of Horsepower" to the Institution of Civil Engineers,
wherein he noted that he had been studying the performance of horses in
hauling rock from excavations. He observed that a horse worked at
33,000 ft-lbs/min was likely to drop dead by end of a shift. One
respondent to the paper pointed out that Mr. James Watt was in the
business of selling steam engines. He sold them on the basis
of "satisfaction or your money back". And, the respondent added, "being
a Scotsman, he was determined never to have to give any money back, so
defined a horsepower as being a rate of work no average horse could
achieve", thereby ensuring that none of his customers ever had grounds
for complaint regarding the advertised power of his engines.

Best wishes,

Larry
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Larry Cartwright <exit60@ia4u.net>
Physics and Physical Science Teacher
Charlotte HS, Charlotte MI USA
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Tim O'Donnell wrote:

My textbook states Watt basically said a horsepower is the rate of
work a horse can do for a sustained period of time. Any clue to what
would be considered a sustained period of time?


brian whatcott <inet@intellisys.net> Altus OK
Eureka!