ae: 
w 
222 On Kirkwood’s law of the rotation of the Primary Planets. 
IV. The values of C computed for Jupiter and Saturn do not 
harmonize as well as ought to be expected if Kirkwood’s law is 
indeed a law of nature; but upon this I will not insist, as the 
computed sphere of attraction of Jupiter depends upon the mass 
and distance of the next interior planet, with regard to which 
there is considerable uncertainty. 
I will now proceed to examine some of the analogies already 
alluded to, as existing between the planets of our system. We 
shall find that the planets are naturally divided into two classes, 
each distinguished by peculiar characteristics. This will appear 
from the following table, in which, column second exhibits the 
true diameter of each of the planets, and column third their den- 
sity, (the earth being taken as unity ;) column fourth shows the 
time of rotation upon their axes, and column fifth the compres- 
sion of each planet as far as it has been determined. 
epee OT RS! Oo Oth A Treeadiguieter. Density. _| Rotation. _|Compression. 
metedry, oe OBE bh g8 1-225 ZAh-] 
mene Og ee 0-975 0-908 23 ‘3 
meen 8 SP SE EON 1-000 24 0 530 
aNreys,;- 0-517 0:972 24 -6 a7 
eaters! aca 
pupiter se Sp Ss, 10-860 0:227 9 °9 73-7 
Saturn, . 9-982 0-131 10°5 | sy 
Uranus, 4:332 0-167 Ts 
Neptune, A741 0-230 
difficult to measure. If the compression of Venus was the same 
as that of the earth, it would make but one-thirtieth of a second 
difference between its polar and equatorial diameters when in 
Superior Conjunction, a quantity too small to be sensible in micro- 
metric measurements. The compression of Mars has been vatl- 
