C84 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 

 hitih densities, thus : — 



o 



Name of Period of Axial -p, .. 



Planet. Rotation. iJensicy. 



Mercury .... 24 hour3 5 minutes . . . 7 '94. 



Venus 23 „ 21 „ ... 3-33 



Earth 23 „ 56 „ ... 5-67 



Mars 24 „ 37 „ ... 5-84 



A similar table for the otlier larger planets, is as 

 follows : — 



Jupiter 9 hours 55 minutes . . , i'36 



Saturn 10 „ 29 „ ... 74 



Uranus 9 » 3° „ ... '97 



Neptune .... — „ — . . . ro2 



It will be observed that in neither group is the equality 

 of the rotational period or the density more than rudely 

 approximate; nevertheless the difi'erence of the numl)ers 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 witli 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. Lonsr period. Small size. High Density. No satellite.s. 

 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 

 Jufiiter, Uranus, or other planets.^ The classification of 

 nebuhe as commenced by the two Herschels, and continued 



' Philoxophuxd Magazine, 4t]i Series, vol. xx.'cix. p. 396 ; vol. x1. 

 p. 183 ; vol. xli. p. 44. See also Proetor, Fojmlar IScieJice llcviav, 

 October 1874, P- ZS°' 



