On a new genus of Crustacea. 223 
ously estimated. Sir William Herschel states that the polar diam- * 
eter is about ,'. less than the equatorial diameter ; but according 
to Arago, the two diameters are in the ratio of 189 to 194 which 
gives a compression of ,';. Even this compression appears 
irreconcilable with the supposition that the figure of the planet is 
gure of equilibrium of a fluid body. 
The compression of the planets of the second class varies from 
1 
* 
The compression of a planet depends not merely upon its ve- 
locity of rotation, but upon the ratio of its centrifugal force to 
the gravity of the planet, and upon the law of density from the 
surface to its center. The centrifugal force can be computed 
when we know the time of rotation, but the law of density im 
the interior of the planet we have no direct means of determining. 
Here then we find two classes of bodies with characteristics 
plainly marked ; and if we suppose the asteroids to have been 
once united in a single body, probably no one would hesitate to 
assign it to the first of these classes. 
These coincidences are so striking that we seem irresistibly 
led to the conclusion that the time of rotation of Uranus upon its 
axis cannot much exceed that of Jupiter or Saturn ; and it is im- 
probable that the period of the asteroid planet could much exceed 
twenty-four hours. : 
I cannot therefore agree with Mr. Walker in his conclusion 
that “ whether Kirkwood’s analogy is or is not the expression of 
a physical law, it is at least that of a physical fact in the mechan- 
ism of the universe.” Even if future discoveries should prove it 
to be a fact, the law seems irreconcilable with what we must at 
present regard as the most probable values of the planetary ele- 
ments. It is however much to be desired that the periods of ro- 
tation of each of the planets, particularly that of Uranus, should 
be determined with all possible precision, in order that any un- 
certainty which now rests upon this subject may be entirely 
dissipated. 
Ar. XXVI—On a new. Genus of Crustacea in the Collections 
of the U.S. Exploring Expedition under Capt. C. Wilkes, 
U.S. N.; by James D. Dana. 
