B. Hooke — Law of Densities of Planetary Bodies. 395 



two bodies, was the mean of the following values assigned to 

 the earth's surface density by eminent authorities : 



Laplace . - 2*50 



Airy 2-56 



Waltershausen 2*66 



Mean of the above values 2'57 



The values of the mean diameters and mean densities of the 

 earth and moon used in the investigation, are as follows : 



Mean diameter of the earth *7918 miles. 



Mean diameter of the moon 2160 " 



Mean density of the earth _ 5 '66 that of water. 



Mean density of the moon 3*42 " " 



(All of the densities given in this article are relative to that 

 of water, which is taken as 1.) 



Let AB, on the diametral scale, represent the diameter of 

 the moon, and AC the diameter of the earth; draw CD, on 

 the scale of density, for the earth's 

 mean density, BE for the moon's 

 mean density, and AH for the 

 density of a planetary body whose 

 diameter is supposed to equal 0, 

 and also for the surface density 

 of the earth and moon ; draw 11 G 

 parallel to AC, and we have GD 

 for the difference between the 

 mean and surface density of the 

 earth, and FE for the difference 

 between the mean and surface 

 density of the moon. 



Adopting for A II, BE and CD, the values given above for 

 the earth's surface density, the moon's mean density and the 

 earth's mean density, respectively; and for AC and AB the 

 values given for the respective diameters of the earth and 

 moon, we perceive that the point E lies so nearly in a straight 

 line connecting H and D, that it is highly probably that 



FE : GD : : AB : A c. 



From the above simple investigation we conclude that for 

 planetary bodies of the same surface density, the increase of 

 the difference oetween the mean and surface density is propor- 

 tional to the increase of diameter. A test of the correctness 

 of this conclusion will be made by applying the law to the 

 computation of the mean densities of the inner planets from 

 their assigned diameters. 



