316 POri'LAR LECTURES AXD ADD/JESSES. 



precession and nutation of the earth and from the 

 phenomena of the tides. These arguments have 

 been discussed in detail in a paper on the Rigidity 

 of the Earth, 1 and in Thomson and Tait's Natural 

 Philosophy i Vol. I. ; and more recently in my 

 Address to the Mathematical Section of the British 

 Association at its meeting in Glasgow." The argu- 

 ments derived from the phenomena of precession 

 and nutation present considerable difficulties, and 

 indeed do not afford us at the present time a 

 decisive answer. The phenomena of the tides, 

 however, lead us to no uncertain conclusion. 

 Suppose the earth to consist of a thin shell or crust 

 enclosing, or floating on, a vast interior of molten 

 matter. The liquid interior would tend to yield 

 freely to the tide-generating influence of the sun 

 and moon. The consequence would be that the 

 exterior crust would be acted on by forces which, 

 unless it were of prctcrnaturally rigid material (I 

 shall give you numbers directly), it would be unable 



1 "On tin- Rigidity of the Knrlh." W. Thomson, Trans. A'.-V 

 May, i 



()]). 240, 244 above, in the present volume. 



