462 H. Hennessy— Properties of the matter 
supposition that this fluid fills the whole interior, and that the 
solid crust is a mere exterior envelope, is usually designated as 
the hypothesis of internal fluidity. From this hypothesis 
mechanical and physical results of primary importance in terres- 
trial physics may be deduced. 
Newton, Clairaut, Laplace, Airy, and other illustrious mathe- 
maticians have used an extension of this hypothesis in discuss- 
ing the earth’s figure. They supposed the particles composing 
the earth to retain the same positions after solidification as that 
which they held before it. I ventured, for the first time, to 
discard the latter portion of the hypothesis as useless and con- 
trary to physical laws. I now venture to say that, in framing 
any hypotheses as to the physical character of the matter of the 
earth, we should not affix any property to the supposed matter 
which is opposed to the properties observed in similar kinds of 
matter coming under our direct observation. Observation has 
disclosed that liquids are in general viscid, and that they pos- 
compressible than copper or 
. If these general comparative properties of liquids and 
solids are admitted, it follows that in the hypotheses regarding 
the earth’s internal structure we should most carefully guard 
against any assumption directly in contradiction to such pro- 
erti assuming that the earth contained a fluid totally 
evoid of viscidity and internal friction, the late Mr. Hopkins 
attempted to prove the earth’s entire solidity. He only proved 
that it did not contain any of this imaginary fluid; but he by 
no means proved the non-existence of a liquid possessing the 
es eg of viscidity and internal friction common to all 
iquids. In the Comptes Rendus of the Academy of Sciences 
of Paris for 1871 is a paper in which I have given a résumé 0: 
the arguments against Mr. Hopkins’s conclusions as to the 
earth’s complete solidity; and in the subsequent discussions 
. 55 
t “ Remarques 4 propos d’une Communication de M. Delaunay sur 
fournis par l’Astronomie concernant l’épaisseur de la crofite solide 
Comptes Rendns de 0 Inst. France, Mars 6, 187}, p. 250. 
les résultats 
du Globe,” 
alae A aioe 3 1 SO 
