and estuaries, have created opportunities for tide mills which can be made to do certain kinds of work.

With all these sources of power, and possibly others which may be developed, there is no likelihood that our ingenious race will ever be compelled to fall back upon its own muscles, or even to depend exclusively upon animal power. In that distant day when our coal beds and oil fields are exhausted, the sun's rays will still continue to strike the earth. That being the case, trees and other plants will still grow, though wood could scarcely take the place of coal and petroleum. Alcohol can scarcely become as cheap as gasoline has been in the past, but it can be manufactured in considerable quantities from a variety of plants. Again, the rains and the snows will continue to feed our rivers and turn our water wheels. Electrical transmission will enable us to utilize many streams now running idly to the sea, and to distribute the power over wide areas and send it long distances from the streams. Solar engines may be so perfected as to enable us to utilize the inconceivable and inexhaustible flow of energy which comes to us in the form of direct rays from the sun. The winds will continue to blow and push our sails and turn our windmills. And so long as the earth continues to revolve about its axis, the tides will continue to ebb and flow, and these may furnish us considerable quantities of power.

Even if it should happen that none of these sources, nor all of them combined, should furnish power quite so cheap as that which we now enjoy through the use of coal, still we may become so well-to-do, through improved agriculture, improved technical processes for utilizing power, and more rational habits of living, as to enable us to bear the extra cost of these other kinds of power with no great inconvenience. Even if this should not happen, it must not be forgotten that a considerable number of civilizations have been built up and multitudes of people have lived comfortably and happily with no power except that of their own muscles, their domestic animals, the winds, and the waterfalls.

The steam engine. Next to the yoking of the ox at some time in the prehistoric past, the most momentous event in the history of man's power was the invention of the steam engine. The reason why this was so momentous was that the coal beds of the north temperate zone furnish a vast quantity of very cheap and very concentrated fuel. It is difficult to see how the heat of burning coal could have been transformed into mechanical power in any other economical way. The great cheapness and economy of this source of power is what has made it such a powerful factor in the development of modern industry. By merely vaporizing water in a boiler by means of this cheap fuel, enormous pressure can be exerted. This pressure can be made to move a piston. From this point on, further developments are merely the results of mechanical adjustments. Whenever one object, such as a piston, can be made to move as we want it to move, other objects can be hitched to it and be made to move also. The first of these mechanical adjustments to produce great results was when the moving piston was made to turn a wheel, thus converting linear motion into circular motion. After that adjustment was made, every form of steamdriven machinery became a mechanical possibility.

Time does not permit us to mention all even of the really important adjustments which have been made for the greater utilization of the pressure of steam on a movable piston. The economical conversion of mechanical power into electricity, and of electricity back into mechanical power, has enabled us to utilize power in a variety of ways which were formerly impractical, besides giving rise to an electrical industry of vast proportions. The internal combustion engine has made possible automobiles and flying machines.

Roads. The subject of roads and tracks would furnish an interesting study to supplement a study of power. The better the track, of course, the less power it requires to move an object. This would include everything from the air and the ocean, railway tracks, paved streets, and dirt roads, down to the

lubricated grooves, cylinders, and sockets through which the parts of a machine are made to move. Roads, streets, and railway tracks will be discussed under the head of transportation. The rest must be left to the imagination of the student.

Noneconomic properties of land. Some of the physical and geometric properties of land which are the most fundamental are not the most important from an economic point of view. The solidity of the earth which serves to support our weight, and that of the buildings which we erect and the plants which we grow, is of course essential to our very existence. It is not a matter of the greatest economic interest, however, because it is not so scarce as some other properties. Rocky or desert land, of which there is an abundance, furnishes support as well as fertile land. The quality of extension, that is, superficial area, is also essential. It is this which enables us to catch and utilize the sun's rays, the rain, and the dew. It is this which provides room for plants to grow, to spread their roots to the soil and their leaves to the air. It is this which furnishes space for the erection of buildings and the carrying on of all activities. This quality of extension, however, is possessed by sterile as well as by fertile land, and by land which is badly located as well as by land which is well located.

Economic properties. Location may also be said to be a geometric property of land. It is a matter of great economic importance, because there is such a scarcity of land in the best locations. By location is meant proximity and convenience of access to markets, roads, schools, scenery, and various other desirable things. Some land is greatly superior to other land

in this respect, and this creates a great difference in the desirability of different lands. Location is the chief, almost the only factor in determining the value of urban land. In a place where multitudes of people desire to live, land is necessarily scarce, but the scarcity is a scarcity of land well located for urban purposes; that is, for business or for the dwellings of those who have to live within reach of the business establishments. Moreover, the differences in the value of lands within a city are due almost wholly to differences in location. In agricultural communities location is a factor, but not the only nor the most important factor, in determining land values. Nearness to market or to railroads, the character of the wagon roads, accessibility to schools and other social advantages; count for much; but the character of the soil and the subsoil, the climate, the moisture, and the other factors which determine plant growth, count far more. All these factors which promote plant growth may be grouped under the name fertility. In that case we may say that from an economic point of view location and fertility are the most important properties of agricultural land.

Good location saves transportation. When we look for the reason why location is a matter of such importance, we must recall the fact that man's chief work, on the physical side, is the moving of materials. It is this which requires power; and power is costly, whether it be generated in the human body and exercised through the muscles, or whether it be developed in the bodies of animals, or through mechanical agents. One very important phase of the work of moving materials is that of marketing products. The nearer a body of land is to a market, and the better the means of transportation, the less labor and power it takes to get its products to market. On land which is well located with respect to markets it is therefore possible to utilize labor more efficiently than on land which is badly located.

It is also costly to move man himself. It is therefore advantageous that he should live in close proximity to his work.