216 D. Kirkwood on Certain Analogies in the Solar System. 
37 hours. It has been objected, however, that a more rapid rota- 
tion would seem to be indicated by Madler’s determination of the 
ellipticity. In addition to my remarks on this subject in a for- 
mer number of this Journal,* I will merely state that Madler’s 
measurement has not beenconfirmed. ‘ Other astronomers, with 
more powerful telescopes, have not succeeded in gaining any cer- 
tain evidence of an appreciable difference in the diameters.” Mr. 
O. Struve states “ that the grand refractor at Pulkova affords no 
‘indications of ellipticity.” If any thing, therefore, can be infer- 
red from the figure of Uranus, it is a s/ow rotation. 
The objection that the rotation-period of this planet ought to 
be nearly equal to that of Jupiter or Saturn, because of the simi 
larity of the three bodies in other respects, has certainly but little 
of reason or analogy to sustain it. It has been customary to re-.. 
gard the principal planets as arranged in two distinct groups, sepa- 
rated by the region of the asteroids. It has been gtated that in 
magnitude, density, polar compression, &c., there is a striking 
uniformity between the members of each group, and hence it has 
been argued that in order to complete the similarity, we must 
suppose their rotatory velocities to be nearly equal. Now what are. 
the facts in regard to this classification? Let us briefly examine. 
The volume of the Earth or Venus is about seven times that 
of Mars, or nearly twenty times that of Mercury ; while in the 
exterior group, the volume of Uranus or Neptune is less than one- 
tenth that of either of the other two major planets. Jupiter and: 
Saturn are very much compressed at the poles, while the best tel- 
escopes in Europe afford no evidence of the ellipticity of Uranus 
or Neptune. Again: Venus and the Earth have greater masses 
compared with Uranus or Neptune, than these latter in comparl- 
son with Jupiter and Saturn. T'o any one who will compare the 
ag elements other discrepancies cannot fail to present them- 
selves. 
ref 
the system’s formation. Its publication in this connection may 
not therefore be improper. 24 
If we commence with Neptune, the most remote planet know®, 
we shall find that the primary planets are arranged in PatRs, the 
; cap, lee ee 
. ag 
* [2], xi, pp. 394-398. Sir W. a Gnnlon rar ficure of Sat- 
et ee : 98. Sir W. Herschel’s opinion in regard to the gar geet 
nk rroneous. 
+ Hind's Solar System, p. 121, See also Main’s Rudimentary Astronomy, 
24 
