1865.] On Local Attraction. 39 
of the fluid should remain the same. At the time that the crust first 
became sufficiently thick to resist fracture under the strain produced by a 
change in its density—that is, when it first ceased to depend for the 
elevation or depression of its several parts upon the principles of floatation— 
the total amount of matter in any vertical prism, drawn down into the 
fluid below to a given distance from the earth’s centre, had been the same 
through all the previous changes. After this, any further contraction or 
any expansion in the solid crust would not alter the amount of matter in 
the vertical prism, except where there was an ocean; in the case of greater 
contraction under an ocean than elsewhere, the ocean would become deeper 
and the amount of matter greater, and in case of a less contraction or of an 
expansion of the crust under an ocean, the ocean would become shallower, 
or the amount of matter in the vertical prism less than before. It is not 
likely that expansion and contraction in the solid crust would affect the 
arrangement of matter in any other way. That changes of level do take 
place, by the rising and sinking of the surface, is a well-established fact, 
which rather favours these theoretical considerations. But they receive, 
I think, great support from the other fact, that the large effects of the ocean 
at Punne and of the mountains at Kaliana almost entirely disappear from 
the resultant deflections brought out by the calculation. 
_ This theory, that the wide ocean has been collected on parts of the earth’s 
surface where hollows have been made by the contraction and therefore 
increased density of the crust below, is well illustrated by the existence of a 
whole hemisphere of water, of which New Zealand is the pole, in stable 
equilibrium. Were the crust beneath only of the same density as that 
beneath the surrounding continents, the water would be drawn off by 
attraction and not allowed to stand in the undisturbed position it now 
occupies. 
“I have, in what goes before, supposed that, in solidifying, the crust 
contracts and grows denser, as this appears to be most natural, though, 
after the solid mass is formed, it may either expand or contract, according 
as an accession or diminution of heat may take place. If, however, in the 
process of solidifying, the mass becomes lighter, the same conclusion will 
follow—the mountains being formed by a greater degree of expansion of 
the crust beneath them, and not by a less contraction, than in the other 
parts of the crust. It may seem at first difficult to conceive how a crust 
could be formed at all, if in the act of solidification it becomes heavier 
than the fluid on which it rests; for the equilibrium of the heavy crust 
floating on a lighter fluid would be unstable, and the crust would sooner or 
later be broken through, and would sink down into the fluid, which would 
overflow it’ If, however, this process went on perpetually, the descending 
