94 PRESIDENTS ADDRESS—SECTION C. 
medium within confined limits.” And Mr. Fisher, who has quite 
lately supported this theory, thinks that a thin liquid substratum 
exists, which is hotter and therefore less dense under the oceanic 
areas than under the continents, and as the substratum cannot 
be in equilibrium when it is not of equal density at equal depths, 
convection currents take place, the more heated material under 
the oceans flowing towards the continental areas and descending 
there, while new upward currents are started under the oceans. 
The surface currents, he also thinks, tend to carry the crust 
with them and thus compress it. The immediate cause of these 
movements, he says, is the heat of the interior, but he makes no 
attempt to explain why the liquid substratum under the oceans 
should be constantly more highly heated than that under the 
continents, and the hypothesis requires an unequal distribution 
of temperature in the interior which does not appear to be 
possible. 
There is some independent evidence, although it is slight, to 
show that internal movements do actually take place in the earth. 
Professor Newcomb, from some observed irregularities in the 
moon’s motion, infers that the rotational velocity of the earth 
is not constant, but irregular, and he suggests as an explanation 
the flow of a large mass of internal fluid from equatorial to polar 
regions, and wice versa. Also, Captain F. G. Evans, in 1878, 
stated his opinion that certain changes known to have taken 
place in terrestrial magnetism are caused, not by any external 
-agency, but by movements going on in the interior. Mr. Fisher 
finds another reason in the thinness of the earth’s crust, which, 
he says, probably does not much exceed 25 miles. He calculates 
that if the fluid interior was quiescent the crust would have 
attained a thickness of 25 miles in eleven millions of years, and 
as a much longer time than that, he thinks, has elapsed since 
solidification of the exterior took place, there must be some agent 
at work preventing the crust from thickening. This agent he 
holds to be convection currents, which, coming up from below, 
‘melt off the inner layers. 
But assuming that internal currents take place, we have at 
present no adequate explanation of the cause. Professor Phillips’ 
-hypothesis gives no explanation of alternating movements, and 
therefore does not recommend itself as a sufficient cause of oscil- 
lations. The only possible efficient motor seems to me to be 
bodily tides. Humboldt, in his ‘“ Cosmos,” quotes Ampere as 
being of opinion that bodily tides must exercise considerable 
force, and that it was difficult to conceive how the crust resisted 
them. He also says that Poisson allowed the existence of bodily 
tides, but regarded them as inconsiderable, “ as in the open sea 
the effect hardly amounts to fifteen inches.” That some tidal 
deformation must exist is also allowed by Professor Darwin, and 
this tidal deformation will be greatest in the outer layers and 
