ingenious feat of all which our primitive ancestor accomplished was in learning to utilize the muscular energy of other animals. Of course the example was always before him in the observed activity of the animals on every side. Nevertheless, it was doubtless long before the idea suggested itself, and probably longer still before it was put into practise, of utilizing this almost inexhaustible natural supply of working energy.

DOMESTICATED ANIMALS

The first animal domesticated is believed to have been the dog, and this animal is still used, as everyone knows, as a beast of burden in the far North, and in some European cities, particularly in those of Germany. Subsequently the ox was domesticated, but it is probable that for a vast period of time it was used for food purposes, rather than as a beast of burden. And lastly the horse, the worker par excellence, was made captive by some Asiatic tribes having the genius of invention, and in due course this fleetest of carriers and most efficient of draught animals was introduced into all civilized nations.

Doubtless for a long time the energy of the horse was utilized in an uneconomical way, through binding the burden on its back, or causing it to drag the burden along the ground. But this is inferential, since, as we have seen, the wheel was invented in pre-historic times, and at the dawn of history we find the Babylonians driving harnessed horses attached to wheeled vehicles. From that day to this the method of using

horse-power has not greatly changed. The vast majority of the many millions of horses that are employed every day in helping on the world's work, use their strength without gain or loss through leverage, and with only the aid of rolling friction to increase their capacity as beasts of burden.

To a certain extent horse-power is still used with the aid of the modified treadmill just referred toconsisting essentially of an inclined plane of flexible mechanism made into an endless platform, which the horse causes to revolve as he goes through the movements of walking upon it. In agricultural districts this form of power is still sometimes used to run threshing machines, cider mills, wood-saws, and the like. Another application of horse-power to the same ends is accomplished through harnessing a horse to a long lever like the spoke of a wheel, fastened to an axis, which is made to revolve as the horse walks about it. Several horses are sometimes hitched to such a mechanism, which becomes then a wheel of several spokes. But this mechanism, which was common enough in agricultural districts two or three decades ago, has been practically superseded in recent years by the perambulatory steam engine.

It is obvious that the amount of work which a horse can accomplish must vary greatly with the size and quality of the horse, and with the particular method by which its energy is applied. For the purposes of comparison, however, an arbitrary amount of work has been fixed upon as constituting what is called a horse-power. This amount is the equivalent of raising

TWO APPARATUSES FOR THE UTILIZATION OF ANIMAL POWER

The upper figure shows the type of portable horse-power machine used for threshing grain in 1851. The lower figure is an inclined-plane horse-gear. The horse stands on the sloping platform tied to the bar in front, so that it is compelled to walk as the platform recedes.

thirty-three thousand pounds of weight to the height of one foot in one minute. It would be hard to say just why this particular standard was fixed upon, since it certainly represents more than the average capacity of a horse. It is, however, a standard which long usage (it was first suggested by Watt, of steam-engine fame) has rendered convenient, and one which the machinist refers to constantly in speaking of the efficiency of the various types of artificial machines. All questions of the exact legitimacy of this particular standard aside, it was highly appropriate that the labor of the horse, which has made up so large a share of the labor of the past, and which is still so extensively utilized, should continue to be taken as the measuring standard of the world's work.