506 
MR. HENNESSY’S RESEARCHES IN TERRESTRIAL PHYSICS. 
The value of n found from this expression by successive approximations, may be 
substituted in (9.), whence the value of can be obtained. 
By means of the operations above indicated, if sutlicient data existed, some know- 
ledge could be obtained of the arrangement of the molecules of the primitive fluid about 
their centre of gravity. At present no experimental data appear to exist by which 
numerical values could be assigned to c or 1. The experiments of MM. Colladon 
and Sturm show that the compressibilities of different liquids vary considerably, 
according to their physical properties. It would be impossible therefore, from ana- 
logy with their experiments, or those of M. Oersted, to form any accurate idea of 
the compressibility of the fluid constituting the earth when in its supposed state of 
fusion. The calculation of a precise value for n or a, seems then to be at present 
impossible, but geological considerations appear to impose certain limits on the value 
which may have had. The value of may have been equal to, or greater or less 
than the present semipolar axis of the earth. If it were greater, the difference should 
be small compared to its whole length ; for if this difference were great, the external 
shell first formed should (no matter from what cause) arrive at its present dimensions, 
and in doing so it would have to undergo immense contortions of whose existence no 
evidence can be adduced. If a surface be conceived to pass through the centres of 
gravity of the infinite number of pyramidal segments into which we may conceive the 
shell to be divided, it is evident that all of these segments could not descend below 
that surface, or could not all approach in the same degree towards the centre of the 
mass. The shell should be broken in many places to permit the displacement of the 
segments, as the amount of common displacement from the contraction of the shell 
by refrigeration would not effect the reduction of its dimensions to those which it 
has at present. The irregularities of the surface of the fractured shell would evidently 
be in proportion to its primitive axes. 
If were less than the present semipolar axis of the earth, then in order that the 
external coat of the shell first formed should arrive at its present dimensions, it 
should receive additions in some parts. If the difference between and the present 
semipolar axis should have been great, the effect of the contraction of the shell from 
refrigeration may, as in the foregoing paragraph, be neglected. From some cause, no 
matter what, the earth’s mass should expand, and in the expansion rents would be 
produced in the shell. These empty spaces maybe afterwards filled up with injected 
matter pushed forward by further expansion during the subsequent formation of the 
under strata of the shell. The entire shell formed of the cemented portions of the 
first solidified mass, and the new coats of matter added from within, may be again 
ruptured, and a process similar to the first may be again repeated. As the whole 
shell would increase in thickness, the extent and frequency of these ruptures would 
probably lessen, as whatever maybe the mechanical action producing them, it would 
then have an increased resistance to overcome. The ejected matter which would 
arrive at the surface would also probably not amalgamate so completely with the per- 
