§ 9. #The sources of heat, light, and power#. The law of the conservation of energy expresses the fundamental likeness of heat, light, and power. The principal sources from which man derives these agencies are coal and falling waters, tho wood is of importance as fuel in some localities. About 500,000 square miles of land (about 13 per cent of the area of the country) are underlaid with coal. These deposits are widely distributed, so that nearly every part of the country is within 500 miles of a mine. The enormous deposits if used at the present amounts per year would last probably 2,000 to 4,000 years, but if used at the present increasing rate (doubling the product every ten years) they would, it has been estimated, last but 150 years. What shall be the actual rate as between these extremes is a question whose answer depends on our economic legislation as to ownership, exploitation, prices, use, and substitution. This is another of our important socio-economic problems. The one great available substitute for coal as a source of heat and light and power is water power. It is estimated that in 1908 but 5,400,000 horse power was being developed from water falls, whereas about 37,000,000 primary horse power[6] was available; but, by the storage of flood waters so as to equalize the flow, at least 100,000,000 horse power, and possibly double that amount, could be developed. As it requires ten tons of coal to develop one horse power a year in a steam engine by present methods, there is here a potential substitute for coal equal to two to four times our present annual use of coal (about 500,000,000 tons in 1912). But this does not mean that it would be economical, at present costs of mining coal and of building reservoirs, to make this substitution now. To determine when, how far, and by what methods to develop this water power from lakes and rivers for the use of the people and to make this substitution, is another of our great economic problems. Petroleum and natural gas, of which our original reservoirs were perhaps the richest in the world, are being rapidly exhausted. These may be merely mentioned as being related to coal in the source of their supply, in the nature of their uses, and in the economic problems to which they give rise.