2G 
hypothesis requires to give definite value to an unknown law of density and 
to an unknown force of cohesion of parts. The fluid theory is therefore 
more simple. 
Herschel has suggested that if the earth had been originally spherical with 
rotation, the ocean would have covered tke equatorial parts, and left bare 
the polar ; and that the degradation by the sea of the polar land would be 
followed by transfer of sediment to the equatorial regions, and thus a 
protuberance of those regions would result. This theory has been accepted 
by some Geologists. But our author has shown that this would account for 
only such an amount of Precession and Lunar inequality as is less than half 
the actual amount ; and therefore he again insists on the greater probability 
of that hypothesis which assumes the former fluidity of the globe. 
In respect to refrigeration of the globe it may be observed that there are 
two modes of cooling, that of solid bodies by conduction, and that of fluid 
bodies by circulation or convection. If the globe was originally fluid by 
heat, cooling would begin by circulation. But another consideration must 
be taken into account, the tendency to produce solidification by pressure 
which increases with increase of depth. But neither this law of increase 
nor that of increase of temperature is known. The only conclusion is, that 
if the increase of temperature is so rapid as to resist effectually the increased 
pressure, then there will be the greatest tendency to become imperfectly 
fluid, and to solidify towards the surface. Otherwise the opposite result may 
be expected. In the former case the fluidity towards the exterior would 
decrease, until a solid crust was formed, and then heat would pass into the 
planetary spaces by conduction only. In the latter case, the influence from 
pressure is supposed to prevail ; and then the interior would be solid, and 
the solidification proceed towards the surface; the fluidity would diminish 
and a crust be formed, the ultimate state being a solid nucleus and a solid 
crust with intervening fluid. Hence three conclusions may be supposed : 
1. A solid exterior and an internal fused mass, of which the fluidity is 
greatest at the centre. 
2. An exterior shell and a central solid nucleus with fused matter 
between them. 
3. An entire solid globe. 
The author proceeds to refer to his investigations respecting the amount 
of solar and lunar precession on each of these three suppositions, from which 
it appeared that the closest approximation to the actually observed amount 
was gained, on the assumption that the thickness of the solid shell could not 
be less than one fourth or one fifth of the radius of its external surface. 
Other conditions are then discussed. The crust being commenced, would 
