246 Professor TyndaVs Lecture on Force. 



wards the sun. The researches of Sir John Herschel and Mr 

 Pouillet have informed us of the annual expenditure of the sun 

 as regards heat ; and by an easy calculation we ascertain the 

 precise amount of the expenditure which falls to the share of our 

 planet. Out of 2300 million parts of light and heat the earth 

 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 sur- 

 face. Besides, were the sun a burning body merely, its light 

 and heat would assuredly 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 it- 

 self out. What agency can then 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 pro- 

 duced by the combustion of an equal weight of coal. How much 

 greater must be the heat developed by a body falling towards the 

 sun ! The maximum velocity with which a body can strike the 

 earth is about seven miles in a second ; the maximum velocity 

 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 generated 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, and entering 

 our atmosphere with planetary velocity. By friction against the 

 air they are raised to incandescence, and caused to emit light and 

 heat. At certain seasons of the year they shower down upon us 

 in great numbers. In Boston 240,000 of them were observed in 

 nine hours. There is no reason to suppose that the planetary 

 system is limited to " vast masses of enormous weight ;" there is 

 every reason to believe that space is stocked with smaller masses, 

 which obey the same laws as the large ones. That lenticular en- 

 velope which surrounds the sun, and which is known to astron- 



