144 HEAT AND LIGH7" AS COMPANIONS 



a known quantity of water (Fig. 89); at the expiration of a 

 few hours the temperature of the water will be considerably 

 higher. Lampblack is a good absorber of heat, and it is used 

 as a coating in order that all the light rays which fall upon 

 the cylinder may be absorbed and none lost by reflection. 



Light and heat rays fall upon the lampblack, pass through 

 the cylinder, and heat the water. We know that the red 

 light rays have the largest share toward heating the water, 

 because if the cylinder is surrounded by blue glass which ab- 

 sorbs the red rays and prevents their passage into the water, 

 the temperature of the water begins to fall. That the other 

 light rays have a small share would have been clear from the 

 preceding 'Section. 



All the energy of the sunshine which falls upon the cylin- 

 der, both as heat and as light, is absorbed in the form of heat, 

 and the total amount of this energy can be calculated from 

 the increase in the temperature of the water. The energy 

 which heated the water would have passed onward to the sur- 

 face of the earth if its path had not been obstructed by the 

 cylinder of water ; and we can be sure that the energy which 

 entered the water and changed its temperature would ordi- 

 narily have heated an equal area of the earth's surface ; and 

 from this, we can calculate the energy falling upon the entire 

 surface of the earth during any one day. 



Computations based upon this experiment show that 

 the earth receives daily from the sun the equivalent of 

 341,000,000,000 horse power an amount inconceivable 

 to the human mind. 



Professor Young gives a striking picture of what this 

 energy of the sun could do. A solid column of ice 93,000,000 

 miles long and 2\ miles in diameter could be melted in a 

 single second if the sun could concentrate its entire power 

 on the ice. 



