#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.