P. HooJce — Law of Densities of Planetary Bodies. 401 



that of being encircled by a system of rings, we may fairly 

 conclude that the results given above probably represent truly 

 the relative, if not the actual, surface densities of the outer 

 planets and the sun. 



The following interesting inquiry naturally presents itself in 

 considering the law connecting the mean densities of the 

 planets with their diameters : What, under the law connecting 

 the mean density with the diameter, should be the law con- 

 necting the density with the pressure ? The relation of den- 

 sity to pressure, as deduced by the writer from the law of the 

 planets' densities, is, that the increase of the square of the den- 

 sity minus the density, is proportional to the increase of pres- 

 sure • thus, if D is the density and P the pressure, the increase 

 of D 2 — D is proportional to the increase of P. The relation of 

 density to pressure, according to Laplace's hypothetical law of 

 density within the earth, is, that the increase of the square of 

 the density is proportional to the increase of pressure. For 

 very great condensations the two laws would give approxi- 

 mately the same results. 



The writer believes that all eminent observers who have in 

 late years directed their attention to the examination of the 

 surfaces of Jupiter and Saturn, agree that these planets present 

 features widely different from those presented by the inner 

 planets, and that on account of the rapid changes taking place 

 on their surfaces, the interior must be the seat of enormous 

 activity, which can be ascribed to no other cause than to a 

 very high temperature resembling that of the sun. Could the 

 planets Uranus and Neptune be examined from a distance not 

 greater than that which separates us from Jupiter, they would 

 in all probability present features similar to those of the latter 

 planet. We are therefore justified by observation in assigning 

 the outer planets to a different class from that to which the 

 inner planets belong. The adopted hypothesis and the law of 

 density herein indicated, require that the planets Jupiter and 

 Saturn be not only in a fluid condition, but that their present 

 volumes are from 12 to 17 times greater, respectively, than 

 that which they will have after solidification has taken place. 



Of the improbability of the law of density, herein indicated 

 for the inner planets, being true, the writer has not been able 

 to obtain the slightest evidence, based on accurate knowledge 

 (the greatest disagreement in the test to which the law has 

 been subjected, being in the case where there was the greatest 

 uncertainty in the values with which those derived from the 

 law were compared) ; but on the other hand all of the evidence 

 which has any bearing on the subject at all tends to the con- 

 firmation of the law. For instance the most recent changes in 



Am. Jour. Sci.— Third Series, Vol, XXXVIII, No. 227.— Not., 1889. 

 26 



