The radiant energy received from the sun is the source of practically all the forces that man directs upon earth. The winds blow because the sun heats different portions of the earth’s atmosphere unequally. There are waterfalls because the sun has evaporated and raised up into the air enormous quantities of water. When coal and petroleum burn they give up energy that plants received from the sun in bygone geological ages. When a man uses his muscle, or his brain, he is directing energies that were received by plants from the sun, perhaps transformed by some other animal before he uses them. Although the stored-up energies received from the sun in far-off geological ages may eventually be exhausted, there are still possibilities of having plenty, for our planet is receiving energy at the rate of 160,000 horse-power per inhabitant of the earth at the present time.