Mechanical Horse-Power.*
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What It Is, and How It Is Measured.
The term " horse-power " as applied to a mechanical prime mover, i.e., a steam, water, air, or gas engine, dates back to the penultimate decade of the t8th century, and it is almost needless to say that the term was first applied in the case of steam engines. Newcomen's type of steam pumping engine, as applied to mining work, had been in use since about 1710. But it was not until the famous association of Boulton and Watt, three-quarters of a century later, for the production of steam engines on a business scale, that the necessity arose for any standard of power measurement. In those days, about 178o to Ino, Matthew Boulton and Watt were making a commercial success of their steam engines, which were among the earliest in general use. As these engines were mostly supplied for driving mills, pumps, etc., which hitherto had been worked by horses, it became a matter of commercial convenience to rate the engines in terms of the horses they were expected to replace. Consequently, Boulton and Watt set themselves to find out what was the average power of a horse in ordinary work. They took for their type the strong dray horse used in the service of the London breweries. Why they selected this type of horse I am unable to say. Perhaps they were unusually honest and wished to show that the steam horse-power they provided-was much superior to the animal variety which they replaced. Anyway, they did .elect the London dray horse, and they found that, working for eight hours a day, a horse could travel at the rate of 4 miles an hour, and, from experiments of lifting weights attached to a cord led over a pulley, they also found that the average which a horse could lift steadily was isolb. Applying these two results and assuming that their average horse would continue to lift isolb. while travelling at its usual rate for an hour, they deduced the average power of a horse as that required to raise rsolb. through 4 miles in an hour, or db. through 33,000 feet in the minute; or, conversely, to raise 33,00olb. through r foot in the same time. Thus, the mechanical unit of horse-power became 33,000 foot-pounds per minute, and, misleading as the figure is when applied in comparison with animal horse-power, it is nevertheless now universally accepted by all British engineers, and, with only sfight modification, to suit the system of notation of the country, by those of foreign nations also.
The Power of a Horse.
I have said that the term " horse-power " is misleading, and for the following reasons. The mechanical horsepower has an absolutely definite value incapable of variation, while the power of a horse is constantly varying between very wide limits, and varying not only with the type of horse, but also, and more important, with the conditions of the work in which its power is employed. The experiment of Boulton and Watt was, no doubt, a perfectly fair and reasonable one so far as it went, but, whereas the weightlifting test lasted, probably, for a few minutes only at most, they assumed that the horse would continue to work steadily at the same rate for an hour, or, in fact, for a day, since their estimate of speed was based upon eight hours' work. Now we know, from other experiments which have been carried out since Watt's time, that a good strong horse in a short effort, lasting a few seconds, is, perhaps, the equal of as much as 15 ine:iianical horse-power. Also we know that a horse gets tired, and, whatever his power, he cannot exert it after a certain period; in fact, it is steadily decreasing all the time until the horse is no longer able to work. On the other hand, the mechanical horse-power, whilst not
" forceful " as the animal at starting, maintains a constant value indefinitely. That is to say, a 2h.p. engine will continue to work at the rate of two horse-power when the two animals which it may be taken to represent are ready to drop from fatigue and are not capable of exerting anything like their full power for even a fraction of a minute. Con sequently, it will be realised that there can be no actual coinparison in general terms between the power of an engine and that of a horse.
What is Meant by the Term.
Before proceeding further, I would enjoin, a clear conception of what horse-power is. It is.ttot force, which, however, is frequently, though hazily confounded with it.I The definition of force is a somewhat lengthy one, and it is sufficient for our purpose to define it as a cause which produces or tends to produce motion, and it is measured in pounds or other units of weight. It is a force (that of the exploded mixture) which causes the piston to move in the cylinder, but this force may remain constant whatever the number of revolutions of the engine, while the horse-power varies almost directly with the speed. Neither is horse-power the work done by the engine, but it is a measure of the rate at which work is done. Thus we have seen that one horse. power is 33,000 foot-pounds per minute, but, if the work of 33,000 foot-pounds be done in half a minute, then the engine which has accomplished that amount of work has been operating at the rate of two horse-power. " Horse-power " is an equivalent term to " speed," whilst it dilfers from " work " in the same way that " miles per hour " differs from " miles "; also, we shall now clearly understand that when we refer to a 2oh.p. motor we mean a motor which is capable of doing 21) times a definite amount of work in one Min Elk.
Nominal Horse Power.
Besides horse-power pure and simple there are several modifications of the term which have been brought into common use by the methods employed in computing or measuring the rate of work done by engines under various conditions. It was comparatively easy for Boulton and Watt to arrive at a definite figure to represent a horse-power, but it was another matter accurately to measure the horse-power of their own engines, since the " indicator " had not then been • invented. At least, they adopted a system of rating their engines by "nominal horse-power" (N.H.P.), which gave results very different from the actual horse-power. For the purpose of calculation the mean pressure (of steam) on the piston was assumed to be 71b. per square inch, and the average speed of the piston in feet per minute was taken at 128 times the cube root of the length of stroke in feet (L). The product of these, with the piston area (A), was, of course, divided by 33,000 to get the h.p. The formula was
7 x A x (128 x
33,000 It is almost unnecessary to point out that, since the mean pressure and the piston speed-were assumed to be practically constant, when, really, they probably varied considerably for different engines of the same type, the measure of h.p. was scarcely dependable.
As steam pressures increased, in course of progress, the assumed mean pressure on the piston was raised by Bourne to 211b. per sq. in., the other factors remaining as before. Since then the formula has been further amended by engine manufacturers and by the Admiralty, but, as the steam engine was improved and stage expansion adopted (i.e., compound, triple, and quadruple-expansion engines) the relations between nominal and actual horse-power became still more strained, and the introduction of the engine indicator and the measurement of the mean pressure on the piston from the indicator card caused the term nominal horse-power to fall into desuetude. It is still, however, employed by manufacturers of agricultural steam engines, but is never met with now in motor-vehicle work.