SECT. X. DECREASE OF TEMPERATURE. 73 



indeed such plants could flourish without the intense light of a 

 tropical sun. But, even if the decreasing 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 measure of the periods of the celes- 

 tial 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 diminished 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 temperature. That accomplished 

 author, in pointing out the mutual dependencies of phenomena, 

 says, "It is evident that the mean temperature of the whole 

 surface of the globe, in so far as it is maintained by the action of 

 the sun at a higher degree than it would have were the sun ex- 

 tinguished, must depend on the mean quantity of the sun's rays 

 which it receives, or which comes to the same thing on the 

 total quantity received in a given invariable time ; and, the 

 length of the year being unchangeable in all the fluctuations of 

 the planetary system, it follows that the total amount of solar 

 radiation will determine, ccrteris paribus, the general climate of 

 the earth. Now, it is not difficult to show that this amount is 

 inversely proportional to the minor axis of the ellipse described 

 by the earth about the sun (N. 143), regarded as slowly vari- 

 able ; and that, therefore, the major axis remaining, as we know 

 it to be, constant, and the orbit being actually in a state of 

 approach to a circle, and consequently the minor axis being on 

 the increase, the mean annual amount of solar radiation received 



