By land the economists understand not only the firmer portions of the earth's surface but the whole surface, water as well as what is ordinarily called land. Land, as the economist understands it, means also the atmosphere which surrounds the earth as well as the minerals and other useful things found under the surface of the earth. It includes, moreover, not merely the materials which nature furnishes to man but also the chemical and mechanical and vital forces which nature places at man's disposal. It includes the power which resides in the natural waterfalls. It includes the heat from the sun's rays which ripens the grain. In a word, the term land is used to designate nature's contribution to the productive process.