Prof. Tyndall on Force. 61 



from the earth that the attraction of the earth is barely sensible, 

 and let it fall to the earth from this distance. It would reach the 

 earth with a final velocity of 36,747 feet in a second; and on col- 

 lision with the earth the body would generate about twice the 

 amount of heat generated by the combustion of an equal weight of 

 coal. We have stated that by falling through a space of 16 feet 

 our lead bullet would be heated three-fifths of a degree ; but a body 

 falling from an infinite distance has already used up 1,299,999 parts 

 out of 1,300,000 of the earth's pulling power, when it has arrived 

 within 16 feet of the surface ; on this space only 130 ^ 0Qo th of the 

 whole force is exerted. 



Let us now turn our thoughts for a moment from the earth 

 towards the sun. The researches of Sir John Herschel and M. 

 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 pro- 

 duce the temperature of the sun's surface. 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 com- 

 bustion 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 re- 

 garded 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 

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

 strike the earth is about 7 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 



