PRESIDENT’S ADDRESS —SECTION ©. T7 
Still later, Professor Darwin says that the lunar 19-yearly tide 
is the only one by which we can hope to test the rigidity of the 
earth. He calculates that this tide would give an oscillation of 
1% inches at the poles, and about half that amount at the equator. 
He endeavoured to ascertain whether this tide could be detected 
in the observations made at Kurachi, but found that, if it existed 
at all, it was completely masked by tides due to meteorological 
causes. He concludes that “we must regard it as extremely 
improbable that the 19-yearly tide will ever be detected,” and, 
consequently, that ‘ the evaluation of the earth’s rigidity appears, 
with present data, to be unattainable.” 
In the second of Mr. Hopkins’ papers, already mentioned, he 
arrived at the conclusion that the earth must at one time have 
consisted of a solid crust, resting on an imperfectly fluid and 
highly incandescent interior; but, in 1876, Sir W. Thomson 
denied this, and said that if the interior were fluid the solid crust 
must break up by its own weight and sink into it. This opinion 
was based on the supposition that solid rock is more dense 
than when melted; but experiments have since shown that this 
is not always the case; and, in 1878, Sir W. Thomson, with 
remarkable candour, abandoned this argument also. Professor 
Hennessy has also pointed out that if the interior increases in 
density downwards an outer crust could not sink into a lower 
stratum, especially as in all probability it would be more or less 
vesicular. 
That the crust of the earth floats on a liquid substratum, and 
is therefore in a state of hydrostatic equilibrium, was the opinion 
of Cordier in 1827, and of Professor G. Belli, of Pavia, in 1850, 
as it was also of both Sir J. Herschel and Sir G. Airy. The 
latter, in 1855, showed that a continental plateau one hundred 
miles broad and two miles high could not be supported by a solid 
globe of the materials we know at the surface, and that, conse- 
quently, it must float by what he called a root. The equilibrium 
need not be exact, although it must be within the limits of 
breakage of the rocks forming the plateau. Again, in 1878, he 
said that the form of the earth was not such as would be taken 
by a solid structure, but such as would be taken by a fluid mass 
with solids floating on it. There is, however, an objection to the 
idea that continents and mountains float by solid roots, which is 
that, if the interior gets hotter the further we leave the surface 
these solid roots would be pushed downward into hotter layers 
and would melt. It may be that in mountain ranges this melting 
below would be compensated by erosion above ; but this would 
not apply to ancient plateaux like Canada and Scandinavia. 
The question was again investigated by Professor Darwin in 
1879. He says that if the earth had a figure of equilibrium 
appropriate to rotation there would be no dry land, for the 
surface of the solid would correspond with that of the ocean. 
