684 THE PRINCIPLES OF SCIENCE. [CHAP. 



resemble each other very closely in the period of axial 

 rotation, and the same four planets are all found to have 

 high densities, thus : 



Name of Period of Axial T>o-tt, 



Planet. Rotation. 



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



Venus 23 21 ... 5-33 



Earth 23 56 ... 5-67 



Mars 24 37 ... 5-84 



A similar table for the other larger planets, is as 

 follows : 



Jupiter 9 hours 5 5 minutes . . . I '36 



Saturn 10 29 ... 74 



Uranus 9 30 . . . -97 



Neptune .... . . . 1*02 



It will be observed that in neither group is the equality 

 of the rotational period or the density more than rudely 

 approximate ; nevertheless the difference of the numbers in 

 the first and second group is so very well 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 exceptions, the following 

 correlations hold true : 



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

 Exterior Short Great Low Many 



These coincidences point with much probabilty to a 

 difference in the origin of the two groups, but no further 

 explanation of the matter is yet possible. 



The classification of comets according to their periods 

 by Mr. Hind and Mr. A. S. Davies, tends to establish the 

 conclusion that distinct groups of comets have been 

 brought into the solar system by the attractive powers of 

 Jupiter, Uranus, or other planets. 1 The classification of 

 nebulae as commenced by the two Herschels, and continued 



1 Philosophical Magazine, 4th Series, vol. xxxix. p. 396 ; voL xL 

 p. 183 ; vol. xli. p. 44. See also Proctor, Popular Science Review, 

 October 1874, P- 3S- 



