210 D. Kirkwood on Certain Analogies in the Solar System. 
Art. XXIV.—On Certain Analogies in the Solar System ; by 
Professor Dante Kirxwoop, of Delaware College. 
‘I. The Rotations of the Planets—When we compare the dis- 
tances of the planets with their times of orbitual revolution, we 
cannot fail to observe that a greater period always corresponds to a 
greater mean distance—suggesting at once the important fact, that 
the former varies according to some function of the latter. This is 
the great truth which underlies the celebrated third law of Kepler. 
The case is very different, however, when we come to compare 
the periods of rotation. Here no order or connectiof is at first ap- 
arent. Jupiter, the first planet exterior to the asteroids, is the 
largest in the system, and has the shortest period of rotation. Mars, 
the next inéerior to this group, is, with the exception of Mercury, 
the smallest of the eight principal planets, and is the /ongest in com- 
pleting its diurnal revolution. Saturn, whose diameter is rather 
less than that of Jupiter, has a longer time of rotation; while 
Venus, whose volume is somewhat less than that of the Earth, 
revolves on its axis in a shorter period. Again: Venus, the 
planet next interior to the earth’s orbit, and Mars, the first ev- 
terior, have nearly the same density ; while the rotation period 0 
the one is forty minutes Jess, and of the other forty minutes greater 
than that of our planet. 'Thus whether we compare the masses, 
volumes, or distances of the planets, we find the same apparent 
independence in regard to their rotations. This fact has been n0- 
ticed by various writers. ‘“ We know as yet,” says Humboldt, 
“of no inherent necessity, no mechanical, natural law, similar to 
the. one which teaches us that the squares of the periodic times are 
proportional to the cubes of the major axes, by which the above 
named six elements* of the planetary bodies and the form of their 
orbit are made dependent either on one another, or on their mean 
distance from the Sun.”+ “There is no apparent reason,” re- 
marks another distinguished writer, “why this globe should turn 
on its axis just three hundred and sixty-six times while it de- 
scribes its orbit round the Sun: The revolution of the other plat- 
ets, so far as we know them, do not appear to follow any rule by 
which they are connected with the distance from the sun. ter 
cury, Venus, and Mars have days nearly the length of ours. 
Jupiter and Saturn revolve in about ten hours each. For any 
thing we can discover, the earth might have revolved in this oF 
any other smaller period; or we might have had, without ~~ 
chanical inconvenience, much longer days than we have.”t 
= Viz.: “Their absolute size, density, period of rotation, eccentricity, 
ions of their orbits and the axes.” smos, vol. i, p-7 
+ Whewell’s Astronomy and General Physics, p. 27. mh 
ind the in- 
qs ioe 
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