68 DECREASE OF TEMPERATURE. SJCCT. X. 



Previous to the formation of ice at the poles, the 

 ancient lands of northern latitudes might no doubt have 

 been capable of producing those tropical plants pre- 

 served in the coal-measures, if 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 celestial motions, it has been proved that if the 

 length of the day had decreased by the three-thou- 

 sandth part of a second since the observations of Hippar- 

 chus two thousand years ago, it would have diminished 

 the secular equation of the moon by 4"'4. It is there- 

 fore 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 geo- 

 logical 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 observa- 

 tions. 



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 varia- 

 tion of the eccentricity of the terrestrial orbit an evident 

 cause of decreasing temperature. That accomplished 

 author, in pointing out the mutual dependencies of phe- 

 nomena, says, " It is evident that the mean temperature 

 of the whole surface of the globe, in so far as it is main- 

 tained by the action of the sun at a higher degree than 

 it would have were the sun extinguished, must depend 

 on the mean quantity of the sun's rays which it re- 

 ceives, 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, cceteris 

 paribus, the general climate of the earth. Now, it is 



