This is a unit of force introduced by Watt to enable him to determine what size of engine to send to his customers to supersede the number of horses which the new power was to replace. Watt ascertained that the average force exerted by the strongest horse in one of the London breweries was sufficient to raise 83,000 lbs. one foot high in a minute, thus, an engine of 200 horse power would be a force equal to that of 200 horses, each lifting 88,000 lbs. one foot high per minute. In modern practice, however, owing to various modifications and allowances made for friction and other things, the term horse power has ceased to have so definite a meaning. It is made now to refer rather to the size of the cylinder than to the power exerted, and the valne af the unit has been so varied that a horse power may imply 52,000 lbs. or 60,000 lbs. or 66,000 lbs. raised one foot high per minute.1

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