MR. HENNESSY’S researches IN TERRESTRIAL PHYSICS. 
533 
If the effect of refrigeration in contracting the matter of the entire spheroid be now 
considered, it is evident that it will tend to lessen the mean radius of the entire mass, 
and consequently to increase its angular velocity of rotation. The ellipticities of the 
strata of fluid surrounding the nucleus would be greater after every new addition to 
its mass than they were before that addition took place, and therefore, after the com- 
plete solidification of the entire mass, its strata would increase in ellipticity from its 
centre to its surface more rapidly than the strata of the original fluid mass increased 
in ellipticity. In this case, the variation of gravity at the surface of the earth in going 
from its equator to its poles, should, by the theory of the attraction of spheroids, be 
greater than for the same mass in a fluid state with the same ellipticity at surface. Ob- 
servation however shows that, on the contrary, the variation of gravity at the earth’s 
surface is less rapid than for the primitively fluid spheroid. The earth could not 
therefore have solidified in this manner required by this theory. 
16 . If, by the action of refrigeration and compression, solidification proceeded 
simultaneously from the surface towards the centre, and from the centre towards the 
surface, it will not be doubted but that the former process, when once commenced, 
must proceed far more rapidly than the latter. The temperature at the centre would 
be nearly constant, compared with the temperature of the surface of the fluid, from 
the obstacles opposed to convection and the laws of propagation of heat by conduc- 
tion. Solidification at the centre will therefore be due chiefly to pressure ; but if con- 
traction accompany the change of state of the fluid matter in becoming solid, it ap- 
pears from Section III. that the pressure on every stratum of the fluid will decrease 
with the radius of the shell’s inner surface. The pressure on the solid nucleus will 
thus be continually diminishing, while its temperature will remain almost unchanged. 
The solid nucleus would therefore, instead of acquiring increased magnitude, tend to 
return to its original fluid state. This will evidently be true at every period of the 
existence of the nucleus, and hence we must conclude that this mode of solidification 
is incompatible with our original assumptions. 
VII. THE DIRECTIONS OF THE FISSURES IN THE SHELL WHICH MIGHT BE PRODUCED 
BY THE ACTION OF THE PRESSURES CONSIDERED IN SECTION I. 
17. If the pressure of the nucleus against any part of the shell be sufficiently in- 
tense to produce a fissure, the direction of that fissure will depend on those of the 
tensions resulting from the fluid pressure and on the physical structure of the shell. 
In the actual case of the earth it is probable that the highly crystalline structure, 
which we have reason to believe is characteristic of the shell, would give to some 
portions of it a tendency to fracture in particular directions. We have at present no 
precise physical reasons for thinking that this tendency should follow any general 
law, so that it must now be considered as only a source of irregular deviations in the 
directions of the fissures from those which they would have if the shell possessed an 
uniform cohesive strength. 
3 z 
MDCCCLI. 
