The energy which the sun radiates goes in every direction. However, only a minute part of it falls on the earth. Even so, it represents the power of about five million horsepower per square mile per day. The sun gives us as much energy every minute as mankind uses in a year. At present, we use this energy indirectly, and it is our only final source of power. Coal represents the chemical action of the sun on green plants thousands of years ago. Water power results from the sun's creating vapor and the resulting rain. Even windmills operate because of air currents set in motion by the uneven heating effects of the sun in different places. Some day, through chemistry or some type of solar motor, we shall harness this source of energy more directly. Already, a scientist has worked out an engine, surprisingly efficient. This engine uses a series of mirrors which concentrate the sun's energy on a tube of water to create steam.

Q. Underline the sentence which proves that much energy is available from the sun.