Address to the Ro youl Geological Society. 11 
and of the present sun, physical laws often emerge into view, 
and sometimes even into being which before were unsuspected. 
The estimates given of the amount of potential heat in the 
solar system nebula are exceedingly interesting and suggestive ; 
they are very instructive as illustrations of certain ascertained 
principles and undeniable mathematical relations; but as demon- 
strations of the past and present circumstances and actualities of 
the solar system they must be, at any rate, very uncertain, and 
may be, for all we can tell, considerably wide of the mark. 
B.—The next argument we have to notice is that drawn from 
the shape of the earth in connexion with her present rate of 
rotation. Our globe is an oblate spheroid in consequence of the 
centrifugal force of rotation; and if she were fluid, and the law 
of increase of density, in descending towards the centre, were 
known, it would be possible to calculate what would be the 
spheroidicity due to her rate of rotation. Now the actual sphe- 
roidicity of the earth, taken as a whole, agrees pretty nearly 
with that calculated on a reasonably assumed law of increase of 
density. A few years ago the only conclusion that could be 
drawn from this was that the supposed, must agree fairly well with 
the actual, variation of density. But two striking discoveries 
arrived at in late years have made this agreement very im- 
portant in a different way altogether. Delaunay has pointed 
out (originally, as regards himself, though it had been already 
suggested by Kant) that the earth’s rate of rotation must be di- 
minishing, from the action of the ocean tides; and that conse- 
quently the amount of her spheroidicity, also, must be lessening, 
supposing her to be fluid. But Sir William Thomson has con- 
cluded that the earth must be, as a whole, about as rigid as con- 
tinuous steel; and ne argues that as the earth’s shape is nearly 
that corresponding to her present rate of rotation, she must have 
become rigid when her rotation rate was but little higher than it 
now is, that is to say, a comparatively short while back. Prof. 
Tait believes that this argument, taken along with that which 
we have yet to consider, reduces geological time to something 
less than ten millions of years. 
The following may be urged in reply. In the first place, 
granting that the earth is as rigid as steel (when tested as Sir 
William tests her), she will still, as he calculates, yield to the 
