MR. hennessy’s researches in terrestrial physics. 
535 
sure, the maximum and minimum resultant tensions would be nearly equal, and con- 
sequently the directions of the fissures would be governed chiefly by accidental 
causes. Taking into consideration the causes which have been abstracted, it appears 
therefore that the directions of the fissures might in general have no necessary rela- 
tions with that of the equator. If such a fissure commenced forming in the direction 
of any great circle, it would evidently continue to be propagated in the same general 
direction unless accidental causes should alter its course. 
At some stages of the shell’s existence the relation between the variable and the 
constant pressure might be such as to cause the maximum tension at any point to be 
in the direction of the tangent of the meridian, or parallel at that point, and hence, in 
this case at least, all simultaneously formed fissures should be parallel ; the term 
parallel being used to mean that the tangents to the circles forming the prolon- 
gations of these fissures should all form equal angles at the points of intersection with 
the tangents of a great circle bisecting them. 
VIII. ON THE existence OF A ZONE OF LEAST DISTURBANCE IN THE SHELL. 
18. During the process of the formation of the shell, the forces tending to fracture 
it, and those holding its parts together, will in general be continually varying. If the 
intensity of the former class of forces increased much more rapidly than that of the 
latter, there would be no limit to the disturbed part of the shell ; if, on the contrary, 
the latter class of forces increased more rapidly in intensity than the former, some 
parts of the shell might after a certain time be comparatively undisturbed. It is even 
possible to conceive, if the constant pressure of the fluid against the shell be small, 
that in some parts of the shell the cohesive forces keeping its particles together might 
be always greater than the rupturing forces at these parts. 
In order to form a general idea of the positions of such undisturbed portions of the 
shell, the variation of the effective pressure at any point in the shell must be con- 
sidered. Of the two different ways in which the effective pressure could vary in going 
from the equator to the pole of the shell, it is evident, from Section IV., that the only 
one which it is necessary to consider is that of its decrease. In this case we may 
conclude, from article 2, that the pressure everywhere between the equator and 
parallel of mean pressure must be greater than that which exists at the same parallel, 
and that everywhere between this line and the pole the pressure will on the contrary 
be less. The disturbed portion of the shell must therefore be near the equator, and 
that line must divide it into two equal parts. In order that an undisturbed zone of 
the shell may exist, it is not necessary that the latter should be perfectly rigid ; it 
may be capable of subsiding by the abstraction of a certain amount of the pressure 
of the fluid until its parts should rest in equilibrium. With a considerable variable 
pressure and a constant pressure insufficient to produce fractures in the shell, it is 
possible to conceive how this could occur, and hence the most general idea which can 
be formed of the undisturbed portion of the shell at either side of the equator, is that 
3 z 2 
