SECT.X.] INVARIABILITY OF ROTATION. 81 



masses projected by volcanoes from the equator to the poles, 

 and the contrary, would indeed affect it, but there is no evi- 

 dence of such convulsions. The disturbing action of the 

 moon and planets, which has so powerful an effect on the 

 revolution of the earth, in no way influences its rotation. 

 The constant friction of the trade winds on the mountains 

 and continents between the tropics does not impede its ve- 

 locity, which th'eory 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 pro- 

 ducts of the mass of each into its angular velocity and dis- 

 tance 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 tempera- 

 ture may be, when their distances from the centre are di- 

 minished, their angular velocity must be increased, in order 

 that the preceding quantity may still remain constant. It 

 follows, then, that, as the primitive momentum of rotation 

 with which the earth was projected into space must neces- 

 sarily 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. 

 Notwithstanding 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 corre- 

 sponding to the depth and the form of the spheroid being 

 what theory assigns to a fluid mass in rotation, concurs to 



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