1884. 



113 



[Kirkwood. 



The Outer or less dense Group. 



In the following table the rotation periods of Saturn, Uranus and Nep- 

 tune are derived from that of Jupiter by formula (4;. 



It will be noticed that the theoretical period of Saturn is 31 minutes less 

 than Hall's evaluation. 



The Inner Group. 



Planet. 



Mars . . . . 



Earth 



Venus . . , 

 Mercury 



0.1056 

 1.0000 

 0.7690 

 0.0522 



R 



764,650 m 

 1,082,147 

 700,208 

 152,000 



A 



7.446.40 

 24,880.5 

 70,129.2 

 468,616.0 



365.26 

 224.70 



87.97 



Here the rotation periods of the earth, Venus and Mercury are derived 

 from that of Mars by formula (4). The first is two minutes less than the 

 true period ; the time of Venus's rotation is doubtful ; and the theoretical 

 determination of Mercury's period agrees with the estimate of Mr. Den- 

 ning. 



PROC. AMER. PHILOS. SOC. XXTI. 118. O. PRINTED MARCH 5, 1885. 



