MR. HENNESSY’S RESEARCHES IN TERRESTRIAL PHYSICS. 
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be regarded as a coefficient affecting of such a nature as to satisfy the equation 
3. In order to arrive at a solution of the problem of the figure of the earth, it has 
been found necessary to suppose that the primitive fluid mass deviated but little from 
the spherical form. If the spheroid be in rotation, it must follow that its axis of rota- 
tion should be that one of all the axes passing through the centre of gravity, with 
respect to which the sum of the forces acting upon the system would be a maximum. 
This axis may therefore be supposed to coincide very nearly with the position of the 
present axis. If no other forces be supposed to act upon the mass but the mutual 
attractions of its molecules and centrifugal force, then equation (1.) will rigorously 
represent the general equation of its surface, and the consideration of its physical or 
mechanical changes could be treated with less difficulty. 
Whatever may be the cause of the primitive fluidity of the earth, and whatever may 
be the nature of the process of solidification by which at least a portion of it has 
arrived at a solid state, if that process should be such that a shell would be first 
formed surrounding a nucleus of matter in a state of fluidity, and if solidification be 
accompanied by a change in density of the matter solidified, it appears that some 
change would be produced in the motions of the entire mass about its axis, and in 
the arrangement and composition of that portion of the fluid constituting the nucleus. 
The manner in which, from physical or chemical causes, the solidifying process may 
proceed, will to a great extent determine what may be the actions mutually exercised 
by the nucleus and shell. The combination of such actions, with the changes which 
may have occurred in the velocity of rotation of the mass, will be thus known, and 
their influence upon the geological phenomena occurring during the different epochs 
of the physical history of the earth can be thus appreciated. In this way it may be 
possible to carry our ideas back to the remoter periods of the earth’s existence, and 
to assist in explaining the phenomena which geological observers are constantly un- 
folding. 
The application of physical and mechanical science to questions connected with 
geology, has been made only within a comparatively recent period, and the subject 
seems yet to require a more complete systematizing in order to bring it into the class 
of sciences which belong properly to what may be called the mechanics of the uni- 
verse. An important step towards connecting geology with physical astronomy has 
already been made by Mr. Hopkins in his researches concerning the constitution and 
thickness of the present solid shell of the globe. One of the objects of the present 
memoir will be fulfilled if it should in any way tend to increase that connexion. 
4. Adopting in its greatest generality the hypothesis that the earth was originally 
a fluid mass in rotation, the particles of which attracted each other with forces vary- 
ing inversely as the squares of their distances, and that the figure of the mass was 
nearly spherical, it will be impossible to proceed with the general investigation of the 
