Ae 
308 Sir David Brewster’s Address 
_ thenope.* If these eleven small planets are really the remains of a 
larger one, the size of the original planet must have been consider- 
able. / What its size was, would seem to bea problem beyond 
the grasp‘of reason. But human genius has been permitted to 
triumph over greater difficulties. 'The planet Neptune was discov- 
ered before a ray of its light had entered the amen eye; an 
a law of the solar system just discovered, we can determine the 
original magnitude of the broken planet isu after it has been 
shivered into fragments,—and we might have determined it even 
after a single fragment bad proved its existence. This law we 
-owe to Mr. Daniel Kirkwood, of Pottsville, a humble American, 
who, like the dlustdibhs Ss olui struggled to find something new 
among the arithmetical relations of the planetary elements. - Be- 
tween every two adjacent planets there is a point where their at- 
tractions are equal. If we call the distance of this point on one 
side of a planet to that on the other, the diameter of its sphere of 
attraction, then Mr. Kirkwood’s law is, that ih every planet the 
‘Square of the length of its year, reckoned in days, varies as the 
cube of the diameter of its sphere of attraction. .This law has 
been verified by more than one American astronomer, and there 
can be no doubt, as one of them expresses it, that-it is at wie 
a little liter than. Mars, or about. 5,000 mites in Faiaineter a 
of i ust have been about 574 hours. The 
e 
hypothesis of Laplace ;, but we venture 
to say that this opinion will not be adopted by the astronomers 
of England. Among the more recent ceptor within — 
ds of our own Syeibel et te ion th 
our inguished pacited becniey r. La 
* Ceres 1801, Januaty 1 i, .c.seccececsee 
Pallas 1802, March $ 
uno 1804, Septemberl ...... 
Vesta 1807, March 
Astrea ...5%,.2%--ss-++ 1845, December 8 
ss A OS OS SOOT SUNT co caent s 04 
i 1847, August 13 
Flora 1847, October 18 
otis He April 25 
~M . : 
Hygeia .. 849, April 12 
Parthehope............ 1850, May 11 
