270 
GILBERT. 
of my discourse will be devoted chiefly to the development 
of the moonlet theory, and I hope to show that it not only 
harmonizes with the varied details of crater character, but 
aids in the explanation and even in the discovery of other 
features of the moon’s face. 
The idea that a planet or satellite may be developed from 
a ring of matter revolving about the primary has been 
entertained by so many students of celestial mechanics that 
its introduction needs no defense. The assumption that the 
ring about the. earth was thin and disk-like rests legiti¬ 
mately on the analogy of Saturn’s ring. The idea that the 
ring, although possessed of sufficient stability to assume a 
definite form, nevertheless suffered some disturbance or 
underwent some process of evolution by which its stability 
was destroyed, is likewise familiar to celestial mechanics, 
and it does not appear necessary in this connection to specu¬ 
late as to the precise manner in which the integration of its 
discrete elements was effected, nor does it appear necessary 
to assume, on the one hand, that the aggregates of ring 
matter constituting the larger moonlets were loosely as¬ 
sembled, or, on the other hand, that they were already 
welded into continuous masses. Provided the energy of 
impact with the moon sufficed to produce phenomena of 
fusion, the dynamic results would perhaps not differ greatly. 
The velocity of impact, depending chiefly on the moon’s 
attraction, must be supposed to have increased gradually as 
the moon grew. In the closing stages of the process it did 
not vary greatly on either side of one and one-half miles 
per second, and the phenomena of the present surface may 
be discussed on the basis of that velocity. The energy due 
to that velocity would more than suffice, as already stated, 
to melt the moonlet if it were composed of ordinary vol¬ 
canic rock and provided all of the energy were applied to 
the heating of the moonlet. Practically only a portion of 
it was thus applied; another portion produced heat in the 
contiguous tract of the moon’s material; yet another was 
consumed in the deformation of moonlet and moon result- 
