2(U Prof. H. Hennessy on the Properties of the 



posing 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 contrary 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 possess what has been designated internal fric- 

 tion in a high degree*. Observation has recently shown that 

 among the three states of matter (gaseous, liquid, and solid) 

 a law of continuity exists. Observation also discloses that 

 gases and vapours are, of all forms of matter, the most com- 

 pressible, that liquids are much less compressible, and that 

 solids are still less compressible. Thus, for instance, water is 

 about fourteen times more compressible than copper or brass. 

 2. 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- 

 perties. By assuming that the earth contained a fluid totally 

 devoid 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 

 properties of viscidity and internal friction common to all 

 liquids. In the Comptes Rendus of the Academy of Sciences 

 of Paris for 1871 is a paper in which I have given a re'sumeoi 

 the arguments against Mr. Hopkins's conclusions as to the 

 earth's complete solidity; and in the subsequent discussions 

 my priority on this matter seems to have been fairly and 

 honourably acknowledged!. In a recent admirable work on 

 Geology, PfafPs Grundriss der Geologie, the author gives a 

 brief account of the bearing of astronomical and mathematical 

 investigations on the internal structure of the earth; and he 

 very justly says that the results of observation compel us to 

 regard the earth as for the most part fluid, in order to bring 

 these results into harmony with calculation. Professor Pfaff 



* As having a special connexion with this subject, see a Report by the 

 Author on Experiments on the influence of the molecular condition of 

 fluids on their motion when in rotation and in contact with solids (Pro- 

 ceedings of the Ptoyal Irish Academy, 2nd series, vol. iii. p. 55). 



t " Iiemarques a propos d'une Communication de M. Delaunay sur les 

 resultats fournis par l'Astronomie concern ant Tepaissseur de la croute 

 solide du Globe/' Comptes Rendus de Vlnst. France, Mars 6, 1871, 

 p. 250. 



