CLASSIFICATION. 357 



Name of Period of Axial 



Planet. Rotation. Density. 



Mercury .. .. 24 hours 5 minutes .... 7-94 



Venus 23 21 .. .. 5-33 



Earth 23 56 .... 5-67 



Mars 24 37 .. .. 5-84 



Forming a similar table for the other chief planets, it is 

 as follows : 



Name of Period of Axial 



Planet. Rotation. Density. 



Jupiter .. .. 9 hours 55 minutes .. .. i'36 



Saturn .. .. 10 29 .... '74 



Uranus 9 30 .... -97 



Neptune .. .. .... 1*02 



It will of course be observed that in neither group is 

 the equality of the rotational period or of the density more 

 than rudely approximate, nevertheless the difference of 

 the numbers in the first and second group is so very 

 marked, the periods of the first being at least double and 

 the densities four or five times those of the second, that 

 the coincidence cannot be attributed to accident. The 

 reader will also notice that the first group consists of the 

 planets nearest to the sun, that with the exception of 

 the earth none of them possess satellites, and that they 

 are all comparatively small ; the second group are furthest 

 from the sun, and all of them possess several satellites, 

 and are comparatively great. Therefore, with but slight 

 exception, the following correlations hold true : 



Interior planets. Long period. Small size. High density. No satellites. 

 Exterior ,, Short ,, Great Low Many 



These coincidences certainly point with much proba- 

 bility to a difference in the conditions of origin of the 

 two groups, but no further explanation of the matter is 

 yet possible. 



The classification of comets by Mr. Hind and Mr. A. S. 

 Davis according to their periods tends to establish the 

 conclusion that distinct groups of comets have been 



