THE MOON ? S FACE. 
261 
periphery, the remaining part being widely distributed 
through flow of the softer material below. It is possible, 
therefore, to interpret the quantitative relations discovered 
on the moon in terms of local physical condition without 
rejecting the impact theory. 
A fourth difficulty is connected with the circular contours 
of the craters. If a ball of mud be allowed to fall vertically 
upon a horizontal surface of the same material, the result¬ 
ing crater is circular; but if instead it be thrown obliquely, 
the resulting crater has an oval contour. Except for irregu¬ 
larities which may he counted as details of form, some of 
the lunar craters are as nearly circular as can be determined 
by measurement; others are slightly elliptic; a few only are 
notably elongate. It is inferred that the predominant direc¬ 
tion of the incident bodies supposed to have formed them 
was vertical to the lunar surface or nearly so; but it can be 
shown from simple geometric considerations that the pre¬ 
dominant angle of incidence of swift-moving meteoric bodies 
approaching from all directions would be 45 degrees, and 
the scars produced by such collisions would be predom¬ 
inantly oval instead of predominantly circular. So far as 
my reading has extended I have discovered but one sugges¬ 
tion for the obviation of this difficulty, and that was applied 
only to very small lunar craters. It was suggested by 
Proctor that immediately after the shock of collision there 
might be an elastic return to a circular form.* The idea 
requires for its realization a high tensile elasticity, such as 
we do not know in any rocks, but only in certain substances 
of organic origin, and it thus fails to receive support from 
the phenomena of our terrestrial experience. There has 
occurred to me an entirely different mode of escaping the 
difficulty, and as this is my personal contribution to the 
subject it will be set forth somewhat fully. 
Moonlet Theory .—Besides the nomadic and apparently 
individual meteors of space, there are certain groups sym¬ 
metrically arranged and moving in a systematic and orderly 
* The Moon : London, 1873, p. 346. 
