68 AN INTRODUCTION TO SCIENCE 



winter a depth of a few feet only in England the tem- 

 perature steadily rises to the extent of i F. for every 50 or 

 60 feet. This proves that the more superficial strata are 

 losing heat which they receive by conduction from strata 

 placed more deeply. The earth is shedding heat into space. 

 Lord Kelvin has calculated the amount of heat which is 

 dissipated yearly, and has estimated the time which has 

 elapsed since the surface of the earth was so hot that all 

 water upon it must have been in the form of steam. This, 

 he says, was the condition of the globe less than 100 million 

 years ago. 



Lastly, the physicist attacks the problem from quite a 

 different side. Having determined the outside limit of the 

 age of the earth, he turns to the sun and asks, How old is 

 that ? How long has the sun been pouring forth the force 

 which keeps plants and animals alive ? What is the source 

 of his energy ? It cannot come from the same source from 

 which we commonly obtain it, combustion. Had the whole 

 sun been made of coal with an infinite atmosphere of oxy- 

 gen in which to burn, it would have gone out in a few 

 thousand years. When this fact was recognized it was sug- 

 gested that the great mass of the sun might attract me- 

 teors, fragments of broken-up worlds, which would rush 

 towards it with such velocity as to set free, when they struck 

 it, the energy which the sun disperses as heat. But for the 

 supply of the sun's heat in this way meteors equal in size, 

 in the aggregate, to the moon would need to be sacrificed 

 every year, and astronomy proves that space is not pervaded 

 by such a multitude of shooting stars. It is now agreed that 

 the heat of the sun is produced by the collision of the parti- 

 cles of matter of which it is itself composed. These collisions 

 are brought about by the shrinking of the sun, which is 

 losing four miles in diameter every century. To the ques- 

 tion, how long has the emission of heat by this process 

 been going on ? Lord Kelvin answers : ' ' The sun may 

 have already illuminated the earth for as many as 100 million 



