PRESIDENTS ADDRESS—SECTION C. 75 
doned.” The theory was, however, revived by Laplace and by 
Cordier, who, in his “ Essay on the Temperature of the Interior 
of the Earth,” 1827, brought forward numerous reasons for 
thinking that sufficient heat to melt rock must exist at no great 
depth from the surface, and, consequently, that the earth consisted 
of a solid crust surrounding a still melted interior. The reasoning 
was supported by Fourier, and, although opposed by Poisson, 
was rapidly accepted by geologists as the only explanation of 
volcanoes, lateral movements of the crust, and subsidence of the 
surface, all of which seemed to necessitate a yielding interior. 
But this theory received a severe blow in 1839, when Mr. 
W. Hopkins published the first of his “ Researches in Physical 
Geology,” in which he maintained that the amount of precession 
and nutation of the earth’s axis proved the earth to be either 
solid or to have a solid crust not less than 800 miles thick. He 
was supported in this by Archdeacon Pratt, and afterwards by 
Sir W. Thomson ; but their conclusions were opposed by Professor 
Hennessy in 1851, by M. Delaunay in 1868, and subsequently by 
General Barnard and Professor Newcomb. In 1876 Sir W. 
Thomson abandoned this argument altogether, and stated his 
opinion that a perfectly fluid spheroid would have a precession 
practically the same as that of a perfectly rigid one, and Professor 
Darwin has shown that this is so. 
A far more important objection to the theory that the interior 
is fluid is the supposed absence of bodily tides. This subject was 
discussed in the days of Laplace and Cordier, was again taken up 
by Sir W. Thomson in 1863, and has since been supported by 
Professor G. H. Darwin. Sir W. Thomson said that, if the 
interior were fluid, the sun and moon would produce bodily tides, 
which would raise and lower the solid crust, and thus reduce the 
amount of the ocean tides. He calculated that if the earth was 
as rigid as steel, it would yield about two-fifths, and if as rigid as 
glass, more than three-fourths as much to the tide-producing 
influences as if it was fluid; and as the latter amount could, he 
thought, be easily observed if it occurred, he arrived at the con- 
clusion that the earth must be more rigid than glass. Professor 
Hennessy objected to this conclusion, and pointed out that the 
conditions on which Sir W. Thomson’s calculations were based are 
very different from the real ones. He said that if the fluid 
interior passes gradually into the solid crust, as it probably does, 
deformation of the shell may be very small, for the tidal energies 
would be expended in pushing aside the half-liquid matter. He 
also pointed out that, as the observed amount of precession is 
about six seconds less than that calculated for a rigid globe, some 
slight tidal deformation actually takes place. The Rey. O. 
Fisher, also, in the second edition of his “‘ Physics of the Earth’s 
Crust,” 1889, has suggested that if the liquid interior contains 
gas in solution, the elasticity of the gas might compensate for 
