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XXI. The Figure and Primitive Formation of the Earth, or Researches in Terrestrial 
Physics. — Part 1. By Henry Hennessy, M.R.I.A. 8 ^ c . Communicated hy 
Major Ludlow Beamish, F.R.S. 
Received October 29, — Read December 17, 1846. 
1. The hypothesis by which the figures of the heavenly bodies are theoretically 
explained has long engaged the attention of geologists and geometers, without having 
acquired any important improvements. Great as have been the discoveries which have 
originated from this hypothesis, it remains in nearly the same philosophical position 
as that to which it had arrived when Clairaut published his Theory of the Figure of 
the Earth. During the period of more than a century which has elapsed since the 
appearance of that immortal work, the improvements which have been made in the 
theory of the figures of the planets, appear to consist chiefly in discoveries connected 
with the attractions of bodies, and in some important generalizations of the equili- 
brium and motions of fluids. 
Comparatively few positive discoveries have however been as yet made in geology 
as deductions from the hypothesis of the primitive fluidity of the earth. In explana- 
tion of this circumstance several causes may be assigned, one of which appears to be 
the limited nature of the hypothesis. It has not been considered sufficient to suppose 
that the earth was originally a mass of heterogeneous matter in a fluid state; an 
additional supposition has been made, in support of which I am not aware that any 
evidence has been ever adduced. The supposition alluded to is, that the volume of 
the entire mass and the law of density of the fluid have not been changed by the soli- 
dification of a part of that fluid, no matter how far the solidification may have pro- 
ceeded ; or in other words, that the distribution of the molecules composing the 
earth is the same in the present state of our planet as that which they had when the 
mass was fluid. Although no precise evidence can be brought forward for the exami- 
nation of this portion of the hypothesis, it appears not to be entirely consistent with 
what is known respecting the solidification of fluids. It is thus defective from appear- 
ing to involve an unproved and improbable physical law. Its exclusion from the 
hypothesis will make the latter more general and more applicable to the explanation 
of certain cosmical phenomena. Making the simplified hypothesis the basis of my 
investigations, it shall be their object further to generalize the theory of the figure of 
the earth, and to examine the nature and the energy of the physical and mechanical 
actions which may have been exerted upon its surface during its different geological 
transformations. 
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