4.30 TRANSACTIONS OF SECTION A. 
its parts. The earth is in a state which is described technically as a state of 
‘initial stress.’ In the erdinary theory of the mechanics of deformable bodies a 
body is taken to be strained or deformed when there is any stress in it, and the 
strain is taken to be proportional to the stress. This method amounts to measur- 
ing the strain or deformation from an ideal state of zero stress. If the ideal state 
is unattainable without rupture or permanent set or overstrain, the body is in a 
state of initial stress. The commonest example is a golf-ball made of indiarubber 
tightly wound at a high tension. Now the problem of gravitational instability can 
be solved for a planet of the size of the earth on the suppositions that the density 
is uniform and the initial stress is hydrostatic pressure. If the resistance to com- 
pression is sufficiently small the bedy is unstable, both as regards concentration of 
mass towards the centre and as regards displacements by which the density is 
increased in one hemisphere and diminished in the other. A planetary body of suf- 
ficiently small resistance to compression could not exist in the form of a homogeneous 
sphere. It could exist in a state in which the surface is very nearly spherical, and the 
mass is arranged in a continuous series of nearly spherical thin sheets, each of con- 
stant density ; but these sheets would not be concentric. They would be crowded 
together towards one side and spaced out on the opposite side somewhat in the manner 
shown in fig. 4. The effect would be a displacement of the centre of gravity away from 
the centre of figure towards the side where the sheets are crowded together. How 
small must the resistance to compression be in order that this state may be assumed 
hie Fig.5. 
by the body instead of a homogeneous state? The answer is that, if the body has 
the same size and mass as the earth, the material must be as compressible as 
granite. Granite,as we know it at the earth’s surface, is not a typically compressible 
material. A cube of granite 10 feet every way could be compressed from its 
yolume of 1,000 cubic feet to a volume of 999 cubic feet by pressure applied to 
every part of its surface ; but according to the recent measurements of Adams and 
Coker the pressure would have to be rather more than two tons per square inch. 
A homogeneous sphere of the same size and mass as the earth, made of a material 
as nearly incompressible as granite, could not exist; it would be gravitationally 
unstable. The body would take up some such state of aggregation as that 
illustrated in fig. 4, and its centre of gravity would have an eccentric position. 
Now how would an ocean rest on a gravitating sphere of which the centre of 
gravity does not coincide with the centre of figure? Its surface would be a sphere 
‘vith its centre at the centre of gravity (fig. 5). The oceanic region would be on 
one side of the sphere and the continental region on the other side. It was pointed 
out many years ago by Pratt that the existence of the Pacific Ocean shows that 
the centre of gravity of the earth does not coincide with the centre of figure. 
There is no necessity to invoke some great catastrophe to account for the existence 
of the Pacific Ocean, or to think of it as a kind of pit or scar on the surface of the 
earth. The Pacific Ocean resembles nothing so much as a drop of water adhering 
to a greasy shot. The force that keeps the drop in position is surface tension. 
