242 SPECULATIVE SCIENCE 



into mere vapour, which would extend over the whole planetary 

 system, and evaporate us entirely. It has been thought neces- 

 sary to give the foregoing sketch of the inevitable gradual 

 running down of the heavenly mechanism, to show that this 

 reasoning concerning the sun's heat does not depend on any one 

 special fact, or sets of facts, about heat, but is the mere acciden- 

 tal form of decay, which in some shape is inevitable, and the 

 very essential condition of action. There is a kind of vague 

 idea, when the sun is said to be limited in its heating powers, 

 that somehow chemistry or electricity, &c., may reverse all that ; 

 but it has been explained that every one of these agencies is 

 subject to the same law ; they can never twice produce the same 

 change in its entirety. Every change is a decay, meaning by 

 change a change in the distribution of energy. 



Another method by which the rate of decay of our planetary 

 system can be measured, is afforded by the distribution of heat 

 in the earth. If a man were to find a hot ball of iron suspended 

 in the air, and were carefully to ascertain the distribution in 

 the ball, he would be able to determine whether the ball was 

 being heated or cooled at the time. If he found the outside 

 hotter than the inside, he would conclude that in some way the 

 ball was receiving heat from outside ; if he found the inside 

 hotter than the outside, he would conclude that the ball was 

 cooling, and had therefore been hotter before he found it than 

 when he found it. So far mere common-sense would guide him, 

 but with the aid of mathematics and some physical knowledge 

 of the properties of iron and air he would go much farther, and 

 be able to calculate how hot the ball must have been at any 

 given moment, if it had not been interfered with. Thus he would 

 be able to say, the ball must have been hung up less than say 

 five hours ago, for at that time the heat of the ball would have 

 been such, if left in its present position, that the metal would 

 be fused, and so could not hang where he saw it. Precisely 

 analogous reasoning holds with respect to the earth ; it is such 

 a ball ; it is hotter inside than outside. The distribution of the 

 heat near its surface is approximately known. The properties 

 of the matter of which it is composed are approximately known, 

 and hence an approximate calculation can be made of the 

 period of time within which it must have been hot enough to 



