424 Dr. J. R. Mayer on Celestial Dynamics, 



If we accept Laplace's result, that the length of a day has 

 remained constant during the last 2500 years, and conclude 

 that the earth's radius has not diminished H decimetre in con- 

 sequence of cooling, we are obliged to assume, according to the 

 premises stated, that the mean temperature of our planet cannot 

 have decreased 4?.,, in the same period of time. 



The volume of the earth amounts to 2650 millions of cubic 

 miles. A loss of heat sufficient to cool this mass 4^° would 

 be equal to the heat given off when the temperature of 6,150,000 

 cubic miles of water decreases 1° ; hence the loss for one day 

 would be equal to 6*74 cubic miles of heat. 



Fourier has investigated the loss of heat sustained by the 

 earth. Taking the observation that the temperature of the earth 

 increases at the rate of 1° for every 30 metres as the basis of his 

 calculations, this celebrated mathematician finds the heat which 

 the globe loses by conduction through its crust in the space of 

 100 years to be capable of melting a layer of ice 3 metres in 

 thickness and covering the whole surface of the globe; this 

 corresponds in one day to 7'7 cubic miles of heat, and in 2500 

 years to a decrease of 17 centimetres in the length of the 

 radius. 



According to this, the cooling of the globe would be suffi- 

 ciently great to require attention when the earth's velocity of 

 rotation is considered. 



At the same time it is clear that the method employed by 

 Fourier can only bring to our knowledge one part of the heat 

 which is annually lost by the earth; for simple conduction 

 through terra firma is not the only way by which heat escapes 

 from our globe. 



In the first place, we may make mention of the aqueous 

 deposits of our atmosphere, which, as far as they penetrate our 

 earth, wash away, so to speak, a portion of the heat, and thus 

 accelerate the cooling of the globe. The whole quantity of 

 water which falls from the atmosphere upon the land in one day, 

 however, cannot be assumed to be much more than half a cubic 

 mile in volume, hence the cooling effect produced by this water 

 may be neglected in our calculation. The heat carried off by all 

 the thermal springs in the world is very small in comparison 

 with the quantities which we have to consider here. 



Much more important is the effect produced by active vol- 

 canos. As the heat which accompanies the molten matter to 

 the surface is derived from the store in the interior of the earth, 

 their action must influence considerably the diminution of the 

 earth's heat. And we have not only to consider here actual 

 eruptions which take place in succession or simultaneously at 

 different parts of the earth's surface, but also volcanos in a 



