PHILOSOPHICAL TRANSACTIONS. 
I. On the Mechanical Conditions of a Swarm of Meteorites, and on Theories of 
Cosmogony. 
Jly G. H. Darwin, LL.D., F.R.S., Fellow of Trinity College and Plumian Professor 
in the University of Cambridge. 
Received July 12,—Read November 15, 1888. 
Mr. Lockyer writes In his interesting paper on Meteorites'^ as follows :— 
“ The brighter lines in spiral nebulse, and in those in which a rotation has been set 
np, are in all probability due to streams of meteorites with irregular motions out of 
the main streams, in wdiich the collisions would be almost nil. It has already been 
suggested by Professor G. Darwin (‘ Nature,’ vol. 31, 1884-5, p. 25)—using the gaseous 
hypothesis—that in such nebulse ‘ the great mass of the gas is non-luminous, the 
luminosity being an evidence of condensation along lines of low velocity, according to 
a well known hydrodynamical law. From this point of view, the visible nebula may 
be regarded as a luminous diagram of its own stream-lines.’ ” 
Tlie wdiole of Air. Lockyer’s paper, and especially this passage in it, leads me to 
make a suggestion for the reconciliation of two apparently divergent theories of the 
origin of planetary systems. 
The nebular hypothesis depends essentially on the idea that the primitive nebula is 
a rotating mass of fluid, which at successive epochs becomes unstable from excess of 
rotation, and sheds a ring from the equatorial region. 
The researches of PocHEt (apparently but little knowm in this country) have 
imparted to this theory a precision which was wanting in Laplace’s original 
exposition, and have rendered the explanation of the origin of the planets more 
perfect. 
But notwithstanding the high probability that some theory of the kind is true,| the 
acceptance of the nebular hypothesis presents great difficulties. 
Sir WiLLLAM Thomson long ago expressed to roe his opinion that the most pro¬ 
bable origin of the planets was through a gradual accretion of meteoric matter, and 
* ‘ Nature,’ Nov. 17, 1887. The paper itself is in ‘ Roy. Soc. Proc.,’ Nov. 15, 1887 (No. 259, p. 117). 
t ‘ Montpelliei-, Acad. Sci. Mem.’ 
X [The very remarkable photograph of the nebula in Andromeda, exhibited to the Royal Astronomical 
Society by Mr. Isaac Roberts on December 6, 1888, affords something like a proof of the substantial 
truth of the nebular hypothesis.—G. IT. D. December 19, 1888.] 
MDCCCLXXXIX.—A. U 1.2.89 
