FORCE. 375 



receives one. The whole heat emitted by the sun in a 

 minute would be competent to boil 12,000 millions of 

 cubic miles of ice-cold water. How is this enormous 

 loss made good whence is the sun's heat derived, and 

 by what means is it maintained ? No combustion no 

 chemical affinity with which we are acquainted, would 

 be competent to produce the temperature of the sun's 

 surface. Besides, were the sun a burning body merely, 

 its light and heat would speedily come to an end. 

 Supposing it to be a solid globe of coal, its combustion 

 would only cover 4600 years of expenditure. In this 

 short time it would burn itself out. What agency 

 then can produce the temperature and maintain the 

 outlay ? We have already regarded the case of a body 

 falling from a great distance towards the earth, and 

 found that the heat generated by its collision would be 

 twice that produced by the combustion of an equal 

 weight of coal. How much greater must be the heat 

 developed by a body falling against the sun ! The 

 maximum velocity with which a body can strike the 

 earth is about 7 miles in a second ; the maximum velo- 

 city with which it can strike the sun is 390 miles in a 

 second. And as the heat developed by the collision is 

 proportional to the square of the velocity destroyed, an 

 asteroid falling into the sun with the above velocity 

 would generate about 10,000 times the quantity of heat 

 produced by the combustion of an asteroid of coal of 

 the same weight. 



Have we any reason to believe that such bodies 

 exist in space, and that they may be raining down upon 

 the sun ? The meteorites flashing through the air are 

 small planetary bodies, drawn by the earth's attraction. 

 They enter our atmosphere with planetary velocity, 

 and by friction against the air they are raised to incan- 

 descence and caused to emit light and heat. At certain 

 25 



