Fire and Heat 67 



cult to realize the vast amount of heat radiated by the sun. 

 It is estimated that in the tropics the sun's heat is received at 

 the rate of one fourth of a horse power for each square foot of 

 surface. In general the sun radiates upon one square mile 

 of the earth's surface an amount of energy equivalent to the 

 several millions of potential horse power of the waters falling 

 over Niagara. What must be the total amount of energy 

 radiated by the sun throughout the universe ! Our earth re- 

 ceives only a small fraction, that is, about one two-billionth of 

 the total amount. It is upon this comparatively small fraction 

 of the sun's energy that we are entirely dependent for life, 

 light, heat, and power. It is absorbed by plants and animals, 

 by the land and water of the earth, by the water vapor and dust 

 particles in the air. It is the chief factor in causing evapora- 

 tion and the promotion of air currents. It also determines the 

 amount and distribution of the rainfall and with this, the dis- 

 tribution, kind, and abundance of plant and animal life upon 

 the earth. 



Men have long sought- for ways and means to utilize directly 

 the sun's energy. Progress, however, has been slow and suc- 

 cessful in comparatively few fields. The means which we have 

 to-day are of the simplest kind, such as windows and mirrors 

 to reflect and concentrate the sun's energy. Many uses of the 

 sun's heat are common, such as drying certain food materials, 

 as fruits, vegetables, meat and fish, or in accelerating plant 

 growth in hothouses. Appliances consisting of mirrors have 

 been devised to gather and apply at a desired point the sun's 

 energy for such work as pumping and heating water and oper- 

 ating small electric generators. A solar motor has been made 

 which produced enough steam to pump about 1400 gallons of 

 water a minute. Experiments with solar motors are now being 

 made in the southwestern section of the United States. Such 

 a motor may prove to be one of the means in reclaiming vast 

 areas of now barren land where water is available under the 

 surface, but not generally on it. 



