394 R. Hooke — Law of Densities of Planetary Bodies. 



interior of the earth is subjected, and the difference in the 

 mean densities of the moon and the inner planets, to the 

 different degrees of compression which has been produced by 

 gravity in the interior of their masses. It was clear that, 

 under this hypothesis, all of the bodies referred to would have 

 the same surface density, provided they had reached the same 

 physical condition, that is, the condition of solidification. 



In the selection of the planetary bodies for the purpose of 

 experiment, to determine the law of density, and also the 

 bodies to which the law indicated may be applied as a test of 

 its correctness, it was necessary to arrange the bodies of the 

 solar system into two classes, the first to embrace those in 

 which solidification had taken place, and which, under the 

 adopted hypothesis, would have the same surface density-; and 

 the second, those bodies which had not reached the solid state, 

 and in which there was an expansion of volume beyond that 

 which they would have in the solid condition. It is clear that 

 with the latter class the surface density would probably be 

 different with each body. Therefore the bodies to be selected 

 for the experiment, to determine the law connecting the den- 

 sity with the diameter, and. also the bodies to which the law 

 indicated must be applied as a test, must belong to the first 

 class. Though the law in question, when determined, would 

 not be applicable to the planetary bodies of the second class in 

 their present condition, yet it could be applied to the compu- 

 tation of the ultimate diameters and mean densities of those 

 bodies, that is, the diameters and mean densities they will have 

 when they reach the condition of solidification. 



The planetary bodies which were assumed to belong to the 

 first class are the earth, the moon, and the planets Mars, Yenus 

 and Mercury, and probably also the satellites of the outer 

 planets ; and those belonging to the second class are the outer 

 planets, Jupiter, Saturn, Uranus and Neptune, and also the 

 sun. The evidence to justify the foregoing classification will 

 be referred to in the concluding remarks of this paper. 



In seeking for the law connecting the mean density with the 

 diameter it was clear that no attempt should be made to find 

 the law connecting directly the diameter with the mean den- 

 sity as a whole, but simply with that part of the mean density 

 which was due to compression, or, in other words, to find the 

 law connecting directly the diameter with the difference be- 

 tween the mean and surface densities. This made it necessary 

 to adopt some value for the surface density of the bodies 

 selected for the investigation, which were those in which the 

 greatest accuracy had been attained in the determination of 

 their mean densities and diameters, namely, the earth and the 

 moon. And the value adopted for the surface density of the 



