400 J2. Hooke — Law of Densities of Planetary Bodies. 



Sun 326800 times that of the earth. 



Jupiter TofTFS' " " sun - 



Saturn ^^ 



Uranus ^^ " " 



Neptune ^^ 



We may apply the foregoing values of the ultimate mean 

 densities of the sun and outer planets to the computation of 

 the present surface densities of these bodies ; but the results 

 must be based on the following supposition, namely, that in 

 the contraction of the sun or planet from its present diameter 

 to that assigned above for its ultimate diameter, the mean 

 motion of the molecules at different distances from, and in the 

 direction of the center of the mass, would be proportional to 

 their distance from the center. According to this supposition 

 the ratio of the mean to the surface density will remain con- 

 stant during the contraction. 



Let d be the present mean density of the sun; 



D its ultimate mean density ; 

 S its ultimate surface density, 

 and J5T its present surface density, 



^ dS 1-444X2-58 



then A = - F -~ ~ -=0-0424. 



D 87-69 



Applying the same method of computation to the outer 

 planets we get the following values for their present surface 

 densities : — 





Present surface 



Eatio of mean 





density. 



to surface density. 



Sun 



0-0424 



34-0 



Jupiter 



0-208 



6-6 



Saturn 



0-147 



5-1 



Uranus 



0-365 



3-5 



Neptune 



0-321 



3-6 



These results show that the surface density of the sun is, as 

 would naturally be expected, much less than that of either of 

 the outer planets. They also show that the surface densities 

 of Uranus and Neptune are greater than the corresponding 

 values for Jupiter and Saturn ; this is also what might have 

 been expected, as the former planets being smaller than the 

 latter have made greater progress in the process of condensa- 

 tion. The only anomaly in the above results is in the case of 

 Saturn, which, being smaller than Jupiter, might have been 

 expected to have a somewhat greater surface density, though 

 the results show that the surface density of- the former planet 

 is less than that of the latter. But when it is considered that 

 Saturn presents a most striking anomaly in another feature, 



