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Power needed for moving material objects. It has been pointed out in Chapter IX that man's work, on the physical side at least, consists in moving material objects. For this work the first essential is power. The power first applied was, of course, that which was generated in his own body and exercised through his own muscles. But the secret of the industrial success of modern civilized nations lies in their command of other sources of power rather than in any superior muscularity of their own.

Animal power. The first of these other sources of power which man utilized on a large scale was that of animals which he domesticated and enslaved. They are still one of the most important sources, if not the most important source, of power. There were on the farms of the United States in 1920 about 26,000,000 horses and mules, to say nothing of those in use in the cities and towns. If we add those not on farms it brings the number nearly to 30,000,000. It is not easy to compare the actual working power of a horse with that of the horse-power unit as used in measuring the power of a steam engine; but, assuming that they are equal, it would appear that the total animal power in use in the United States was, until recently, very nearly as great as the total steam and water power used in manufacturing.

Among the animals which have furnished power for man's work may be named the horse, the mule, the ass, the ox, the buffalo, the camel, the elephant, the reindeer, the llama, the dog, and the goat. Of these the most important for the northtemperate zone is the horse, though the ox is a close second.

Originally, in fact until very modern times, the horse was used mainly to carry man himself or loads of material on his back rather than for traction; that is, for pulling or drawing loads. Such traction as he was required to perform was the drawing of war chariots and carriages of state and, later, of carriages and vehicles for the conveyance of travelers. His speed fitted him especially for this work. For the slower and heavier work of plowing, harrowing, and drawing heavy loads of farm produce the ox was long considered superior. In the first place, he was larger and heavier than the horses of that day. His heavy body and short legs and his general anatomy seemed to fit him peculiarly for pulling. He fights by pushing with his head. This seemed to call into play the same muscles, bones, and joints as are used in pushing on the yoke. During the last century or so the horse and the mule have been gradually displacing the ox even in agriculture.

Displacement of the ox by the horse. Two factors have contributed to this change from the ox to the horse and the mule as a source of power for farm work. One is the development of large and heavy breeds of horses of such strength and docility as to fit them as well as oxen for the pulling of heavy loads. The other is the development of farm machinery. All large breeds of horses, however, have been developed in the northwestern parts of Europe; that is, in Great Britain, northern France, Belgium, Holland, and Denmark. Whether this is due to something in the soil or climate or simply to the ability of the people of those countries as animal breeders it is impossible to say. Russia and Hungary also are horse-breeding countries and use horses to a certain extent for traction purposes, but they have not produced such huge draft horses as the other countries mentioned. The United States also is breeding large numbers of heavy draft horses, but we have imported our breeding stock from Great Britain, France, and Belgium. We surpass all other countries, however, in the number, quality, and speed of our trotting horses. The lighter breeds of horses not only lack the weight necessary for drawing heavy loads but

they are also likely to be too nervous and excitable. The United States and Canada, together with the countries which originated the heavy breeds, have pretty generally substituted the horse and the mule for the ox even in farm work.

The mule. Southern Europe and the southern part of the United States have made large use of the mule. This hybrid, combining something of the patience and endurance of the ass with the size and strength of the horse, is admirably adapted to farm work in climates where the huge draft horses of the North suffer from the heat and where the lighter horses of the South are too nervous and excitable for the slow, heavy work on the farm. Even the ass has played a humble though useful rôle by furnishing power to those who could not afford a more expensive animal, such as a horse or a mule.

Both the horse and the mule-even the huge draft breedshave one great advantage over the ox; that is, their more rapid gait. While they cannot trot as well as the lighter breeds of horses, they can trot very much better than the ox and they can walk much faster; and in farm work it is this faster walk which counts.

The factor which has had a great deal to do with the substitution of the horse and the mule for the ox is the increased use of agricultural machinery. This has required power of a superior kind, and the horse has proved to be much better adapted than the ox to the drawing and handling of machinery. This is mainly because of his more rapid gait. When the farmer has his money invested in expensive machinery it is important that he get as much work out of it as possible. He can scarcely afford to allow it to run so slowly as would be necessary if it were drawn by oxen.

Farm machinery. Still another factor which has contributed to this end is the higher wages for farm labor in the countries of northwestern Europe, Canada, and the United States. If a farmer were hiring labor at a very low wage, it would not be so important that he get the greatest possible amount of work out of his hired man. But when labor is expensive the effect is

very much the same as when tools and machinery are expensive. It is thus important that as much as possible shall be accomplished by each laborer. It is therefore better to give him a fast-walking team than a slow-walking team.

Historical importance of the ox. The ox, however, from the most ancient times until quite recently, has been the chief if not the sole draft animal of all the races that have used draft animals at all. His docility and patience, his great strength, the cheapness of his harness, and his ability to find his own living when not at work contributed to make him a most valuable assistant to man in his struggle for the conquest of the earth. In the pulling of the heavy wooden plows and harrows that were in use before the modern steel tools were invented, and the lumbering carts that were in use before modern vehicles were constructed, he enabled men to cultivate the soil on a vastly more extensive scale than would have been possible by human muscles alone. He thus contributed to the production of food for increasing populations of men, and in the end he contributed his own body to help feed them, and his own hide in order that they might be shod. In many parts of the world he is still the principal draft animal for farm work. In southern Europe, southern Asia, and parts of South America one may still see magnificent teams of oxen at work in the fields and drawing carts along the highways. They move with a steadiness and massiveness which give the impression of irresistible power; but they are too slow for most of our hustling Americans, though a good many oxen are used in the rough lands of New England. If we take the whole history of man's use of power, it is probable that the ox has furnished more in the aggregate than any other agency, not excluding coal and steam.

Tropical animals. The Asiatic elephant and the camel are admirably fitted for tropical and subtropical countries, the former in moist and the latter in dry climates. The African elephant has never been domesticated, either because of his fierce and intractable disposition or because the natives of Africa did not care to domesticate him. It is a remarkable fact

that the native African races never domesticate any animal,— not even the zebra, which appears capable of domestication. However, no other race has reduced any animal to domestication since prehistoric times. Either prehistoric man was our superior in this art or else we have not sufficiently felt the need of any more animals. The prodigious strength and the remarkable intelligence of the elephant fit him for a variety of operations besides pulling loads. He requires considerable quantities of coarse fodder such as grows abundantly in warm and moist countries. The great advantage of the camel in dry countries is, of course, his well-known ability to work for long periods without water. He is used in parts of southwestern Asia and northern Africa. The water buffalo possesses qualities almost the opposite of the camel; that is to say, he can work only where water is abundant and easily accessible not only for drinking but for frequent bathing or wetting of the skin. He is a powerful animal and well adapted to working in muddy lands and irrigated rice fields.

In polar regions, where vegetation is scarce, the problem of animal power is a more difficult one. Where moss and lichens abound, the reindeer is a valuable source of power. In the high mountain regions of Peru the llama is used for carrying loads but not for traction. Where forage is not found in sufficient abundance, but where meat and fish can be provided, some carnivorous animal has to be used. The dog is the only one which is sufficiently well domesticated to serve the purpose.

Solar energy. The great physical source of power, so far as . man has been able to develop it, is understood to be the sun. The amount of solar energy which comes to the earth in the form of light and heat is so stupendous as to bewilder the imagination. Its most important service is in the promotion of plant growth and, through plants, of animal growth; but it is also transformed into mechanical power in a number of ways.

In the first place, it vaporizes water. Since the air is heavier than water vapor the latter rises, or, more literally, the air falls through gravitation. When this water vapor reaches high alti

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