ment and importance out of proportion, both to the actual use and to the greatest possible productivity of the soil in each of the national areas mentioned. These cases may be taken as typical of a particular stage of development into which some of the nations of to-day have passed naturally, but which the younger nations of the present, as Argentine, Brazil or Canada, can only approach. The stage of development represented by Britain, Germany or Japan depends on the temporary operation of the sixth and seventh physical factors—the possession of potential energy and useful minerals. Later stages, as will be indicated, may very likely bring once more into prominence the influence of the productivity of the soil in the national territory itself.
Potential Energy.—Under the head of potential energy may be grouped all the natural means of developing mechanical power—coal, running water, wind and even the direct rays of the sun. modern civilization is inseparably associated with the use of two things dependent on potential energy in one form or another: first, the use of heat other than that received direct from the sun; and second, the use of machinery which requires mechanical power either for its making or in its operation. But since heat for any purpose may be secured from mechanical power through the medium of electricity, both needs, for machinery and for heat, hinge on the one question of some single form of potential energy. Coal until recently, at least, has been the one important form of potential energy, for the reason that it furnishes heat directly or supplies power through the medium of steam. These qualities coupled with the possibility of transporting the latent energy to the desired place of utilization, have enabled coal to play perhaps a disproportionate part in directing national development, but its effect may, with some qualification, be taken as indicating the part which mechanical energy, in any form, can play in national evolution.
The value of mechanical power to a nation is best expressed in terms of its equivalent in either animal or man power. One horse power, mechanical energy, may be taken as the equivalent of the power of two average horses or of ten men for a working day of ten hours. A modern steam engine requires not over 2 to 5 pounds of coal for the development of one horse power per hour—a gas engine requires even less—hence a very conservative calculation gives fifty horse power for a working day of ten hours from a single ton of coal. In other words, a ton of coal does in a day the work of at least one hundred horses or of five hundred men. Therefore, one man engaged in mining two tons of coal per day, is producing through the expenditure of one man-power the energy equivalent of a population of 1,000 working men. Calculated on the same basis of values, 80,000 tons of coal produced daily for 300 working days in the year—24,000,000 tons annually—are, in the power they afford, equal to the full energy of a working force