Page:Elementary Text-book of Physics (Anthony, 1897).djvu/269

§ 335] the fall. If this energy be not utilized, it develops heat by friction of the water or of the material that may be transported by it. But water-power is only possible so long as the supply of water continues. The supply of water is dependent upon the rains; the rains depend upon evaporation; and evaporation is maintained by solar heat. The energy of water-power is, therefore, transformed solar energy.

A moving mass of air possesses energy equal to the mass multiplied by half the square of the velocity. This energy is available for propelling ships, for turning windmills, and for other work. Winds are due to a disturbance of atmospheric equilibrium by solar heat; and the energy of wind-power, like that of water-power, is, therefore, derived from solar energy.

The ocean currents also possess energy due to their motion, and this motion is, like that of the winds, derived from solar energy.

By far the largest part of the energy employed by man for his purposes is derived from the combustion of wood and coal. This energy exists as the potential energy of chemical combination of oxygen with carbon and hydrogen. Now, we know that vegetable matter is formed by the action of the solar rays through the mechanism of the leaf, and that coal is the carbon of plants that grew and decayed in a past geological age. The energy of wood and coal is, therefore, the transformed energy of solar radiations.

It is well known that, in the animal tissues, a chemical action takes place similar to that involved in combustion. The oxygen taken into the lungs and absorbed by the blood combines, by processes with which we are not here concerned, with the constituents of the food. Among the products of this combination are carbon dioxide and water, as in the combustion of the same substances elsewhere. Lavoisier assumed that such chemical combinations were the source of animal heat, and was the first to attempt a measurement of it. He compared the heat developed with that due to the formation of the carbon dioxide exhaled in a given