GEOLOGICAL CLIMATE. 295 



repeat the statement of dynamical principles 

 which I gave with experimental illustrations to 

 the Society three years ago ; but may remind you 

 of the chief result which is that, for steady 

 rotation, the axis round 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 by the rotation balance one another. 

 The vast transpositions of matter at the earth's 

 surface or else distortions 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 

 hypothesis accounting for 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 recently communi- 

 cated to the Royal Society of London, and the 

 requisitions of dynamical mathematics for an alter- 

 ation of even as much as two or three degrees in the 

 earth's axis in what may be practically called 



