Change of Climate during Geological Epochs, 129 



which have been experienced by our globe are to be attributed 

 to differences in the distribution of sea and land, changes on 

 the earth's surface of the most extravagant and unlikely character 

 must be assumed to have taken place. Another objection which 

 we have to all these hypotheses which have come under our con- 

 sideration is, that every one of them is irreconcileable with the 

 idea of a regular succession of colder and warmer cycles. 



The recurrence of colder and warmer periods evidently points 

 to some great, fixed, and continuously operating cosmical law. 



We have already referred to the hypotheses of our system 

 passing through colder and hotter parts of space, and of the 

 shifting of the earth's axis of rotation, and have shown that they 

 receive no support whatever from the known facts and principles 

 of physical science. The true cosmical cause must be sought 

 for in the relations of our earth to the sun. 



There are two causes affecting the position of the earth in rela- 

 tion to the sun, which must, to a very large extent, influence the 

 earth's climate; viz., the precession of the equinoxes and the 

 change in the excentricity of the earth's orbit. If we duly examine 

 the combined influence of these two causes, we shall find that the 

 northern and southern portions of the globe are subject to an 

 excessively slow secular change of climate, consisting in a slow 

 periodic change of alternate warmer and colder cycles. 



In a paper read before the Geological Society in 1830 *, Sir 

 John Herschel directed attention to the probable influence of 

 the change in the excentricity of the earth's orbit as a cause of 

 the change of climate during geological eras. But as no trust- 

 worthy calculations had then been made regarding the superior 

 limit of excentricity, he was unable to arrive at any positive re- 

 sults on the subject. It is true that Lagrange had investigated 

 the subject, and had arrived at results which were afterwards 

 found to be almost correct ; but as this geometer had assigned 

 very erroneous values to the masses of the smaller planets, not 

 much confidence could be placed in his results. 



Owing to his not having taken fully into consideration certain 

 conditions which greatly affect climate, Sir John Herschel seems 

 to have been of opinion that the general climate of our globe 

 cannot be much affected by the change in the excentricity of its 

 orbit, and this perhaps is the reason which has led geologists in 

 general to take for granted that the changes in ancient climate 

 cannot be attributed to this cause. 



Both the superior and the inferior limit of excentricity have 

 now been determined by M. Leverrier, and it may be well to 

 examine to what extent climatic changes may be referable to 

 this cause. 



* Transactions of the Geological Society, 2nd series, vol. iii. p. 295. 

 Phil Mag, S. 4. Vol. 28. No. 187. Aug. 1864. K 



