586 EB. W. Hilgard on Mallet’'s Theory of Vuleanicity. 
point, the conclusions of Hopkins or himself. 
The subject has since been taken up by Gen. Barnard 
(Smiths. Contr., No. 240), who, while confirming the results of 
Thomson upon the premises assumed that physicist, also 
shows that there are assumable and admissible conditions upon 
which a fluid nucleus with a moderately thick crust may ex- 
hibit the same constant, or periodically recurrent, amounts of 
precession and nutation, as a solid globe. 
Mallet refers to Thomson's argument in favor of great rigid- 
ity as corroborative of the necessity for assuming a crust of 
great thickness, such as would render it inadmissible to assume 
a direct connection 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. For 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 
coincidences of violent earthquakes with tidal extremes, lately 
observed. 
sion. He denies that Delaunay has shaken, in any important 
imsel 
a case of absolute homogeneity and equilibrium—if then. It is 
certainly incompatible with the demonstration made by Prof. 
f 
aden s: it extremely probable that at a compara- 
th the rigidity of all materials must be seri 
y a high temperature, despite of pressure; OF 
en consider alone the secular loss of heat by 
h must result in a contraction affecting un- 
