The hoe, on the other hand, becomes virtually a lengthened arm, enabling a very slight muscular motion to be transformed into the long sweep of the implement, so that with small expenditure of energy the desired work is accomplished. Similarly, the sledge and the axe lengthen out the lever of the arms, so that great momentum is readily acquired, and with the aid of inertia a relatively enormous force can be applied. It will be observed that a laborer in raising a heavy sledge brings the head of the implement near his body, thus shortening the leverage and gaining power at the expense of speed; but extends his arms to their full length as the sledge falls, having now the aid of gravitation, to gain the full advantage of the long arm of the lever in acquiring momentum. Even such elaborately modified implements as the treadmill and the rowboat are operated on the principle of the lever. These also are mechanisms that have come down to us from a high antiquity. Their utility, however, has been greatly decreased in modern times, by the substitution of more elaborate and economical mechanisms for accomplishing their respective purposes. The treadmill, indeed--which might be likened to an overshot waterwheel in which the human foot supplied the place of the falling water in giving power--has become obsolete, though a modification of it, to be driven by animal power, is still sometimes used, as we shall see in a moment. All these are illustrations of mechanisms with the aid of which human labor is made effective. They show the devices by which primitive man used his ingenuity in making his muscular system a more effective machine for the performance of work. But perhaps the most 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.