168 Prof. D. Kirkwood on the Nebular Hypothesis. 
any degree of inclination to the plane of the ecliptic. Their mo- 
T 
times the distance of the sun from the earth. A still greater 
oes produced in the orbits of the comets of 1770, and of 
The largest 
are from fifty to one hundred and fifty miles in diameter, and 
some are from three to four miles deep. In no case, owevel, 3 
doubtless the phenomena justify the inference that at some Te 
mote epoch in the past history of our satellite, its crust has = 
pa 
in operation during the historical period, sink into insignificance. 
In contemplating these striking characteristics of the 
surface—the traces of ancient igneous activity over the whole 
_ visible hemisphere, and the apparent absence of organization 
it is natural to inquire—what is their physical import? ee 
satellite, in accordance with the nebular hypothesis, has solidifi 
m a gaseous state, the earth having also undergone 
process, the latter, on account of its superior magnitude, W 
require a much longer period than the former to cool down from 
* System of the World, Harte’s Translation, vol. ii, p. 364 
