1874.] The Past History of our Moon. 319 
(relatively), and on the other would leave greater play to the 
effects of contraction. Thus we can understand why the 
signs of volcanic action, as distinguished from the action to 
which mountain-ranges are due, should be far more nume- 
rous and important on the moon than on the earth. 
I do not, however, in this place enter specially into the 
consideration of the moon’s stage of volcanic activity, be- 
cause already, in the pages of my Treatise on the Moon 
(Chapter VI.) I have given a full account of that portion of 
my present subject. I may make a few remarks, however, 
on the theory respecting lunar craters touched on in my 
work on *‘ The Moon.” I have mentioned the possibility that 
some among the enormous number of ring-shaped de- 
pressions which are seen on the moon’s surface may have 
been the result of meteoric downfalls in long past ages of 
the moon’s history. One or two critics have spoken of this 
view as though it were too fantastic for serious consideration. 
Now, though I threw out the opinion merely as a suggestion, 
distinctly stating that I should not care to maintain it as a 
theory, and although my own opinion is unfavourable to the 
supposition that any of the more considerable lunar markings 
can be explained in the suggested way, yet it is necessary to 
notice that on the general question whether the moon’s 
surface has been marked or not by meteoric downfalls 
scarcely any reasonable doubts can be entertained. For, 
first, we can scarcely question that the moon’s surface was 
for long ages plastic, and though we may not assign to this 
period nearly so great a length (350 millions of years) as 
Tyndall—following Bischoff—assigns to the period when 
our earth’s surface was cooling from a temperature of 
2000 C. to 200, yet still it must have lasted millions of 
years; and, secondly, we cannot doubt that the process of 
meteoric downfall now going on is not a new thing, but, on 
the contrary, is rather the final stage of a process which 
once took place far more actively. Now Prof. Newton has 
estimated, by a fair estimate of observed facts, that each day 
on the average 400 millions of meteors fall, of all sizes down 
to the minutest discernible in a telescope, upon the earth’s 
atmosphere, so that on the moon’s unprotected globe—with 
its surface one-thirteenth of the earth’s—about 30 millions 
fall each day, even at the present time. Of large meteoric 
masses only a few hundreds fall each year on the earth, and 
perhaps about a hundred on the moon; but still, even at the 
present rate of downfall, millions of large masses must have 
fallen on the moon during the time when her surface was 
plastic, while presumably a much larger number—including 
