Horse Power , in machinery, a measure by which the capacity of engines is rated, established by Boulton and Watt at 33,000 lbs. raised one foot high per minute. On this basis Watt reckoned the force of his steam engines, and the term has continued in use for want of a better. It is unsatisfactory when applied to a steam engine, as it is apparent that the power of the machine varies with the pressure of steam employed. A small steam cylinder of great strength furnished with abundant boiler room may be made to do the work of a much larger engine with little boiler capacity; and it may also be objected to the use of the word, that it has no reference to the quantity of fuel the engine may consume in working up to the power named. The expression is moreover defective, inasmuch as the work of a horse does not continue in action, as may that of the engine, but is interrupted at intervals of a few hours, the length of which varies with the force exerted. Boulton and Watt allowed in their estimate eight hours as the period of work for the horse.
If the measure then is regarded as anything more than a mere conventional unit and as suggesting an actual comparative estimate, the power of the engine, continuing throughout the 24 hours, should be called three times as great as the number commonly assigned to it. Computations that have been made by different engineers of the average power of horses differ greatly in their results. This is to be expected in consequence of the various modes in which their strength is applied, of the various rates of speed (the effective force rapidly decreasing with the increase of speed), and also of the different qualities of the horses. Watt based his calculations upon the work of the powerful draught horses employed at the London breweries. D'Aubuisson estimated the work done by average-sized •horses in whims or hoisting machines at the mines of Freiberg, working 8 hours out of 24 in two relays of 4 hours each, amounting to 16,440 lbs. raised one foot high per minute, less than half the result of Watt's calculations; while Desaguliers made an estimate of 44,000 lbs., under similar circumstances as to the duration of work.
Smeaton's estimate was 22,000 lbs., and Tredgold's 27,500. Different formulas are given for computing the horse power of engines, but they may be reduced to the simple rule of multiplying the effective pressure upon the piston in pounds per square inch by the velocity of the piston in feet per minute, and dividing by 33,000. (See Steam Engine.) - Horse power is also a name given to various machines contrived to be worked by horses. The common horse whim in use at mines is one of these. It consists of a large drum upon a vertical shaft, which is turned round by horses attached to its horizontal arms. The drum is elevated sufficiently for the horses to pass under the rope, which is wound and unwound by its revolutions. Similar machines are made of cast iron in portable forms, by which toothed wheels or belts are made to drive other machinery. Upon ferry boats the horse power has usually consisted of a revolving circular platform, upon which the horse, generally a blind one, travels, pushing this round under his feet as he draws upon the traces, which are fastened to a fixed object.
For threshing machines, circular saws, etc, machines are used in which the horse works upon a narrow platform supported by endless chains, and carried round two drums; the chains are also supported upon friction rollers.