THERMAL CONDUCTIVITY. 23 



physically, because geologists, perhaps accepting too com- 

 pletely the theory of uniform action throughout the world's 

 history, were led to conclude that the earth must have 

 existed millions of millions of years in order to produce the 

 effects that we find on it. But Sir William Thomson has 

 shown that really this is not the right way of proceeding, 

 but that we must conclude that in past ages there were 

 much more violent differences in temperature, more violent 

 storms, that the actions of nature upon the earth's surface 

 must have been more violent owing to a much greater tem- 

 perature ; and this explains how geologists have assumed too 

 long a period for the existence of the earth. There is very 

 little reason to doubt that the statement of Sir William 

 Thomson is perfectly correct, that the earth cannot be 

 older than 100 million years, if we assume its birth to be 

 at the time when the crust was beginning to form, on its 

 surface. 



There was another very important problem which was 

 attacked by Fourier in his splendid work, which is the 

 case when heat is applied to a body in varying quantities 

 at different times. For example, the earth is receiving 

 heat constantly from the sun, but the quantity of heat 

 which it receives varies in different parts of the surface at 

 different times of the year and different times of the day. 

 Consequently, at any part of the earth's surface we find 

 heat being propagated downwards by the sun's action, the 

 surface is heated, and this constant high temperature is 

 gradually propagated downwards. About noon is the daily 

 maximum heat that it sends down into the earth, but 

 owing to the bad conductivity of the earth's substance, 

 only for a very short time is it perceptible. But the 

 difference between summer and winter is very perceptible 

 to a very great depth, and we find as we descend lower and 

 lower that the maximum of heat in the earth's surface 

 becomes retarded. Thus it takes about one year for the 

 maximum of temperature, that is, the summer temperature, 

 to descend to a depth of 50 feet. At present, at the depth 

 of 25 feet below the surface, it is winter, and at a depth 

 of 50 feet it is only last summer. The retardation of 

 the maximum temperature at different depths is directly 

 proportional to the time, but it also depends on the conduc- 

 tivity of the rocks through which the heat is passing ; 



