72 TEMPERATURE OF THE EARTH. SECT. X. 



influences its rotation. The constant friction of the trade winds 

 on the mountains and continents between the tropics does not 

 impede its velocity, which theory even proves to be the same as 

 if the sea, together with the earth, formed one solid mass. But, 

 although these circumstances be insufficient, a variation in the 

 mean temperature would certainly occasion a corresponding 

 change in the velocity of rotation. In the science of dynamics 

 it is a principle in a system of bodies or of particles revolving 

 about a fixed centre, that the momentum or sum of the products 

 of the mass of each into its angular velocity and distance from 

 the centre is a constant quantity, if the system be not deranged 

 by a foreign cause. Now, since the number of particles in the 

 system is the same whatever its temperature may be, when their 

 distances from the centre are diminished, their angular velocity 

 must be increased, in order that the preceding quantity may still 

 remain constant. It follows, then, that, as the primitive mo- 

 mentum of rotation with which the earth was projected into 

 space must necessarily remain the same, the smallest decrease in 

 heat, by contracting the terrestrial spheroid, would accelerate its 

 rotation, and consequently diminish the length of the day. Not- 

 withstanding the constant accession of heat from the sun's rays, 

 geologists have been induced to believe, from the fossil remains, 

 that the mean temperature of the globe is decreasing. 



The high temperature of mines, hot springs, and above all the 

 internal fires which have produced, and do still occasion, such 

 devastation on our planet, indicate an augmentation of heat 

 towards its centre. The increase of density corresponding to the 

 depth and the form of the spheroid, being what theory assigns to 

 a fluid mass in rotation, concurs to induce the idea that the tem- 

 perature of the earth was originally so high as to reduce all the 

 substances of which it is composed to a state of fusion or of 

 vapour, and that in the course of ages it has cooled down to its 

 present state ; that it is still becoming colder ; and that it will 

 continue to do so till the whole mass arrives at the temperature 

 of the medium in which it is placed, or rather at a state of equi- 

 librium between this temperature, the cooling power of its own 

 radiation, and the heating effect of the sun's rays. 



Previous to the formation of ice at the poles, the ancient lands 

 of northern latitudes might, no doubt, have been capable of pro- 

 ducing those tropical plants preserved in the coal-measures, if 



