Prof. D, Kirkwood on the Nebular Hypothesis. 173 
the satellites of Jupiter, immediately after their formation, did 
not move in a perfect vacuum; the less condensible molecules 
of the primitive atmospheres of the sun and planet would then 
constitute a rare medium, the resistance of which being different 
The occasional rapid variation in the 
of Several members of the group—changes not attributable to 
Sao of distance—would seem to indicate some peculiarity 
‘eir physical constitution. 
_(.) The asteroids already discovered amount to one for about 
ery six degrees of longitude. Their number appears to tm- 
* Harte’s Translation of Laplace's System of the World, vol. ii, p. 367. 
