82 DECREASE OF TEMPERATURE. [SECT. x. 



induce the idea that the temperature of the earth was origin- 

 ally 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 equilibrium be- 

 tween this temperature, the cooling power of its own radia- 

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



Previous to the formation of ice at the poles, the an- 

 cient lands of northern latitudes might no doubt have been 

 capable of producing those tropical plants preserved in the 

 coal-measures, if indeed such plants could nourish without 

 the intense light of a tropical sun. But, even if the decreas- 

 ing temperature of the earth be sufficient to produce the 

 observed effects, it must be extremely slow in its operation ; 

 for, in consequence of the rotation of the earth being a mea- 

 sure of the periods of the celestial motions, it has been 

 proved that, if the length of the day had decreased by the 

 three-thousandth part of a second since the observations of 

 Hipparchus two thousand years ago, it would have dimi- 

 nished the secular equation of the moon by 44"'4. It is, 

 therefore, beyond a doubt that the mean temperature of the 

 earth cannot have sensibly varied during that time. If, 

 then, the appearances exhibited by the strata are really 

 owing to a decrease of internal temperature, it either shows 

 the immense periods requisite to produce geological changes, 

 to which two thousand years are as nothing, or that the 

 mean temperature of the earth had arrived at a state of 

 equilibrium before these observations. 



However strong the indications of the primitive fluidity of 

 the earth, as there is no direct proof of it, the hypothesis 

 can only be regarded as very probable. But one of the 

 most profound philosophers and elegant writers of modern 

 times has found in the secular variation of the excentricity 

 of the terrestrial orbit an evident cause of decreasing tern- 



