42 Mr. E. W. Hilgard on some points in 



mainly in the English press. I think it is to be greatly regretted 

 that the original memoir, very tardily published in the Transac- 

 tions of the Royal Society, should be so difficult of access, that 

 few of those interested are enabled to appreciate the caution and 

 laborious conscientiousness which Mallet has brought to bear on 

 his investigation and discussion of this most complex problem, 

 and to what extent he has himself anticipated most of the ob- 

 jections raised. In calling attention to some apparent omissions 

 in this respect, it may be useful to recall the state of the ques- 

 tion as regards some of the more prominent points at issue. 



The first and most sweeping attack upon the very basis of 

 Mallet's theory comes from Sir William Thomson, in a letter to 

 Mr. Poulett Scrope (Nature, Feb. 1, 1872), in which he calls 

 attention to, and reaffirms the results of his investigation (supple- 

 mentary to that of Hopkins) on the effect which a fluid nucleus 

 and imperfect rigidity of the earth must exert upon precession 

 and nutation, and which led him to the conclusion that, unless 

 the rigidity of the globe as a whole were greater than that of 

 steel, there must ensue a tidal deformation of the solid mass, 

 which would sensibly change the amount of precession. He 

 denies that Delaunay has shaken, in any important point, the 

 conclusions of Hopkins or himself. 



The subject has since been taken up by General Burnard 

 (Smith's Contr. No. 240), who, while confirming the results of 

 Thomson upon the premises assumed by that physicist, also 

 shows that there are assumable and admissible conditions upon 

 which a fluid nucleus with a moderately thick crust may exhibit 

 the same constant, or periodically recurrent, amounts of preces- 

 sion and nutation as a solid globe. 



Mallet refers to Thomson's argument in favour of great rigi- 

 dity as corroborative of the necessity for assuming a crust of 

 great thickness, such as would render it inadmissible to assume 

 a direct connexion between volcanoes and the liquid nucleus. 

 But it is difficult to see how the " preternatural rigidity/' made 

 a postulate by Thomson, could in any manner be compatible 

 with the requirements of Mallet's theory. Eor the latter repre- 

 sents the earth's crust as a congeries of fragments, sustained 

 partly by the contracting liquid nucleus, partly by each other 

 on the principle of the arch — therefore necessarily often locally 

 in a state of unstable equilibrium, and liable to be disturbed 

 by slight outside forces. That the tendency to tidal deforma- 

 tion contributes toward producing such disturbances has been 

 rendered probable by Perrey's discussions, and by the repeated 

 coincidence of violent earthquakes with tiaal extremes, lately 

 observed. 



Thomson's assumption, that the postulated rigidity might 



