MISCELLANEOUS SUBJECTS 463 



the tropical sun, this water will soon commence to boil 

 and give off steam. It is not generally known that the 

 tropical sun, without any concentration, can boil water 

 and make steam, but such is the case. 



The sun throws only light rays to the earth, and unless 

 these light rays are absorbed no heat is produced. The 

 rays first pass through the air and some of them are 

 absorbed by the air and heat it to this extent. However, 

 about two-thirds pass through and strike the earth, and 

 here, again, they are absorbed to a varying extent accord- 

 ing to the colour and nature of the surface they strike. 

 When they strike water, most of them are reflected back 

 into space. When they strike the sands of the desert also 

 most of them are reflected away. And even when they 

 strike the green of the forests great losses by reflection 

 occur. This loss by reflection may be seen by looking 

 at the moon. The sun shines on the moon, the moon 

 reflects again most of the light, and a portion of it 

 reaches us as moonlight. 



Now, however, when the sun's rays strike a black sur- 

 face they are all absorbed and turned into heat. Were 

 the moon painted black, then we should not be able to 

 see it at all. Were the whole earth painted black, all 

 of the rays would be absorbed and turned into heat, and 

 before evening a temperature high enough to melt steel 

 would be reached and all life destroyed. 



We have seen that we can collect the rays of the sun 

 and can absorb them and turn them into heat. Now it 

 remains to show how this can be done so as to get direct 

 power in large quantities. 



The sun shining into the black pan we have mentioned 

 above will produce a temperature of about 250 F., and 

 about four British thermal units of heat per minute per 

 square foot of surface exposed can be created in this 

 manner. But if we catch another square foot of sun- 

 light on a mirror and throw this also into the black pan, 

 then we can produce eight British thermal units of heat 

 per minute and get a much higher temperature, and so 

 on for every additional square foot of sunlight we throw 

 into the pan we get an additional four British thermal 

 units and a correspondingly higher temperature. This 



