ON THE 



INTERNAL STRUCTURE OF THE EARTH CONSIDERED AS AFFECTING 

 THE PHENOMENA OF PRECESSION AND NUTATION. 



' Experience shows that phjsicnl problems of difficulty arc never solved in a 

 satisfactory mauner but after reiterated attempts," says Mr. Ivory, in commencing 

 his Memoir (Phil. Trans., 1839) " On the Conditions of Equilibrium of an Incom- 

 pressible Fluid." 



I quote the remark not only for its general truth but for its special application 

 to the subject before us. If the conditions of fluid eqiuUhn'nm be a problem of 

 so much difficulty that " reiterated attempts" of the great physical investigators- 

 Newton himself leading the way— have been needed, shall no tthe more transcen- 

 dently difficult problem of fluid motion, a fortiori, need repeated attempts with 

 perhaps repeated failures, finally, "in a satisfactory manner," to "solve" if? 



In an address before the British Association, delivered in September last. Sir 

 "Wm. Thomson, alluding to recent occurrences, commencing within the walls ot tiiis 

 very building,^ which had " forced him to give all his spare thoughts ever since 

 t) Hopkins' problem of precession and nutation, assuming the earth a rigid 

 spheroidal shell tilled with fluid;" requests his hearers and his read(>rs to strike 

 out from copies under their control paragraphs '23-31 of iiis paper on "The Rigidity 

 of the Earth" (Phil. Trans., 1863), and 847, 848, 849 of Thomson and Tait's 

 "Natural Philosophy." 



In consequence of the " practical rigidity conferred by rotation" (Addendum to 

 Problems of Rotation," Smithsonian C'ontributions, Vol. XIX, p. 4(5) I had long 

 before (see Notes to pp. 38 and 48) expressed ;(o«-concurrence with the paragi-aph 

 848. It is clear then that ignorance of the " quasi-rigidity induced in a liquid by 

 vortex motion" is not a plea I am entitled to make, even though I have to confess the 

 neglect to recognize its legitimate efl'ects in framing some important conclusions. 



In the paper bearing the title cited at the head of this, after quoting the state- 

 ment of Sir Wm. Thomson (" Rigidity of the Earth"), " That the effective tidal 

 rigidity, and what Ave call the precessional effective rigidity of the earth, may be 

 several times as much as that of iron (which would make the phenomena, both of 



' The Session of the Academy of Sciences, at wliieh tliis was read, was held (April IS, 187") in 

 the office of the Secretary of the Sniitlirtonian Iiislitiitiou. 



1 August. 1877. { ^ ) 



