R. Hooke — Law of Densities of Planetary Bodies. 397 



The agreement of the computed and assigned values is very 

 close in the case of Mars, but not so close in the cases of Yenus 

 and Mercury, yet the differences in the latter cases are prob- 

 ably not greater than the uncertainty 'of the assigned values ; 

 for the density of Yenus as given in the Ninth Edition of the 

 Encyclopaedia Britannica (Art. Astronomy), is 1'03 that of the 

 earth, which reduced to that of water, is 5 83. This value 

 is based on a diameter of 7510 miles. Computing the mean 

 density of Yenus from this value of the planet's diameter, 

 according to the law of density, comparing the result with the 

 assigned value just given (5*83) and then taking the mean of 

 the values given in the two comparisons, we have the follow- 

 ing result : 



Values accord- Values computed 

 ing to law from the assigned 



of density. masses and diameters. 



1st comparison 5 - 5t> 5*24 



2d comparison 5*50 5*83 



Mean of the above values 5*530 5-535 



From the results of the foregoing test of the law of density, 

 as applied to the planets Mars and Yenus, the law may be said 

 to be verified. The apparent discrepancy in the case of Mer- 

 cury does not prove the inapplicability of the law to that 

 planet, as the assigned value of the mass is still somewhat in 

 doubt, and the doubt of the mass taken in connection with the 

 uncertainty in the assigned value of the planet's diameter, may 

 be sufficient to eliminate the entire discrepancy. 



The foregoing tests include all of the planets to which the 

 law of density is directly applicable, but, as before stated, it is 

 quite probable that the satellites of the outer planets belong to 

 the same class of bodies as that to which the inner planets and 

 the moon have been assigned. The test cannot, however, be 

 applied to these bodies for the reason that nothing certain is 

 known of their diameters. The masses of the satellites of 

 Jupiter have probably been determined with sufficient accu- 

 racy to give approximate values of their densities, provided the 

 diameters of the satellites were accurately determined. The 

 observed values of the diameters of these satellites are prob- 

 ably very much in error, as their apparent size would be in- 

 creased by irradiation. But if we suppose the principal error 

 in the observed values to be that due to irradiation, we may 

 get some evidence of the applicability of the law of density to 

 these bodies in the following manner : thus, if the errors in 

 the determination of a set of values for the diameters of these 

 satellites were those due to irradiation alone, the value deter- 

 mined for each satellite would probably be affected by the 



