2G0 DISCUSSION OF THE DESICCATION QUESTION, 



another one, of similar character, namely, that the earth's crust has slid upon 

 its nucleus, thus bringing different portions of the surface into new relations 

 of climate. 



These theories, however, meet with little acceptance on the part of geolo- 

 gists, and still less on that of astronomers and physicists of eminence. The 

 entire weight of mathematical and physical investigation is opposed to the 

 idea that the position of the earth's axis can have changed in any perceptible 

 degree. On this point it is only necessary to quote the statement made by 

 an investigator whose authority cannot be questioned. Sir William Thomson, 

 in a paper on geological climate, published in 1877, thus gives his opinion in 

 regard to the theory in question : "As to changes of the earth's axis, I need 

 not repeat the statement of dynamical principles which I gave with experi- 

 mental illustrations to the Society three years ago ; but may remind you of 

 the chief result which is that, for steady rotation, the axis around which the 

 earth revolves must be a ' principal axis of inertia,' that is to say such an 

 axis that the centrifugal forces called into play b}' the rotation balance one 

 another. The vast transpositions of matter at the earth's surface, or else dis- 

 tortions of the whole solid mass, which must have taken place to alter the 

 axis sufficiently to produce sensible change of the climate in any region must 

 be considered and shown to be possible or probable before any h_ypothesis 

 accounting fur changes of climate by alterations of the axis can be admitted. 

 This question has been exhaustively dealt with by Mr. George Darwin in a 

 paper communicated to the Royal Society of London, and the requisitions of 

 dynamical mathematics for an alteration of even as nuich as two or three 

 degrees in the earth's axis in what may practically be called geological time 

 shown to be on purely geological grounds exceedingly improbable."* 



Not only is the theory of change of climate by alterations in the position 

 of the earth's axis entirely imsupported by astronomical and mathematical 

 researches, but it is in no respect in harmony with the facts developed by 

 geological inquiry. No one has ever been able to fix on any particular point, 

 or series of points, to which the polar axis could have been shifted, either 

 at once or by a series of changes, which would bring about such climatic 

 conditions as have been proved by abundant evidence to have existed during 

 the successive geological ages. This theory may therefore be laid aside as 

 being in no respect a satisfactory one. 



In connection with the second class of theories of geological climates, or 



* Tiaiisiictioiis of the Geolugical Sooii-ty of Glasgow, Vol. V. ]>. 249. 



