220 On Kirkwood’s law of the rotation of the Primary Planets. 
~ “hours to make a rotation upon its axis, the value of C computed 
for Uranus will agree with the mean result derived from Venus, 
the Earth, Jupiter and Saturn. Mr. Walker, by employing some- 
what different data, (Am. Journal, vol. x, p. 22,) has obtained a 
result of thirty-three and a half hours. The question therefore 
presented for our decision is, can we admit that Uranus requires 
from 33 to 37 hours to make one rotation upon itsaxis? I think 
this assumption inadmissible for the following reasons : 
1. Laplace in the Mec. Cel., vol. iv, p. 356, says, “ the time 
of rotation of Uranus is not probably much less than that of Ju- 
piter or Saturn ;” and in the last edition of Herschel’s Astronomy, 
page 648, the time of rotation is given at 91 30™ ? 
_ 2, Asimilar conclusion follows from the observed figure of the 
o planet. Sir William Herschel remarked that the dise of Uranus 
was sensibly flattened ; and Prof. Madler has carefully measured 
the diameters of this planet, which indicate a compression of 5!5, 
or almost exactly one ¢enth, which is fully equal to that of Saturn, 
and greater than that of Jupiter. 
f it be admitted that the figure of Uranus is the figure of equ 
librium of a planet once fluid, (and this will not probably be dis- 
puted by the believers in the nebular hypothesis, to which Kirk- 
wood’s law is thought to have added new probability,) then we 
lanet are 4 « y and 4 ap; where  g represents the ratio of the 
ee ee 
axis, and it probably does not require over ten hours. * 
page 222 : ¥ 
I do not therefore perceive how Uranus can be reconciled with 
Kirkwood’s law, by any admissible assumption with regard to 
the value of its elements. , 
IL. We find a similar discrepancy in. the value of C computed 
for the Asteroids. Arg we permitted ‘in this case to assume ele- 
ments which shall harmonize with Kirkwood’s law?. The re 
re 
