78 PRESIDENT’S ADDRESS—SECTION C. 
As there is dry land on the surface, the interior must be ina 
state of stress, and the materials composing it must be strong 
enough to bear this stress. Ifin any particular place there is a 
stress-difference, and no movement takes place, the materials 
must be at least as strong as matter which would break with 
that stress-difference, and he takes resistance to crushing as 
indicating the strength of the material. The results of his calcu- 
lations show that if the earth is solid it must be at least as strong 
as strong granite ; if there is a crust a thousand miles thick, with 
a gaseous inside, the crust must be stronger than granite ; while, 
if the crust is only two or three hundred miles thick, then it 
must be much stronger than granite. In these calculations the 
earth is supposed to be a homogeneous incompressible elastic 
sphere. If the elastic sphere be very compressible, the stress- 
differences would not be so great. From this Professor Darwin 
infers that the interior of the earth is composed of solid 
substances stronger than granite, for he will not allow the only 
alternative, viz., that it is fluid. 
It seems, therefore, that the only valid argument for a solid 
earth is that derived from the tides. But the forces concerned 
with the production of the oceanic semi-diurnal fortnightly and 
monthly tides are so complicated that the time of high water at 
any place cannot be calculated with precision, and it is not 
certain that any fortnightly and monthly tides exist. Neither 
can the time of high earth-tide be predicted, and consequently 
these tides cannot be used for estimating the rigidity of the 
earth. With the semi-annual tides, and with those of still 
longer period, it is thought that both oceanic and bodily tides 
can be calculated ; but, unfortunately, these tides are so small 
that they cannot be observed, so that here also the means of 
ascertaining whether bodily tides exist are absent. It is how- 
ever thought, rather vaguely, that if the interior were not very 
rigid, the oceanic semi-diurnal tides would be much less than 
they are. But this has not been demonstrated, and as the land 
and the ocean would perform independent oscillations, it seems 
probable that, even if the interior be fluid, the movements of 
the land might be altogether hidden among the great irregularities 
of the ocean tides. On the other hand, it is difficult to believe 
that the highly-heated interior is more rigid than the cooled 
crust, and if it is not more rigid than the crust it must be fluid, 
for in no other way can the continents be sustained. It is also 
impossible to believe that oscillations of several miles in 
amplitude can take place on the surface of a rigid sphere; and 
we shall see presently that these movements of the surface are 
not caused by superficial stress-differences, but by plutonic action 
going on in the interior, which seems to be quite incompatible 
with a solid globe. 
Another vague idea entertained by some geologists is that, if 
