186 GROVE KAEL GILBER T— DAVIS [MEMOI Voi TI xxi' 



of geologists are inclined to explain them by the impact of bodies falling upon the moon — that 

 is, by an essentially astronomical process. This suggests that each group of scientists find the 

 craters so difficult to explain by processes with which they are professionally familiar that they 

 have recourse to a process belonging in another field than their own, with which they are prob- 

 ably imperfectly acquainted, and with which they therefore feel freer to take liberties. How- 

 ever that may be, a perusal of Gilbert's address leaves no doubt that his moonlet theory was, 

 in the first place, invented by an observer who knew full well what terrestrial volcanoes are; 

 that it was, in the second plac'e, based on a critical study of the moon's face; and that it was, 

 in the third place, circumspectly extended into the realms of astronomy in accordance with 

 reasonable physical laws. His summary of the topographic features of the moon is an admirable 

 piece of inductive work. There are a score of dark plains or maria, a score of rugged moun- 

 tain chains, and a few linear troughlike depressions, but of craters there are many thousands, 

 with diameters usually between 10 and 100 miles; yet "through the expression of every feature 

 the lunar crater emphatically denies kinship with the ordinary volcanoes of the earth," hence, 

 as to their origin, differences of form "effectively bar from consideration all volcanic action 

 involving the extensive eruption of lavas," by which terrestrial volcanoes are almost univer- 

 sally characterized. Explosive eruptions, such as are believed to produce the occasional "maare" 

 about in terrestrial volcanic regions, might explain the small lunar pit craters which make 

 one-half of the total number, but not the other half, for maare "lack the wreath [rim], the inner 

 terraces, the inner plain, and the central hill" by which the larger lunar craters are character- 

 ized. An origin by the impact of external bodies is therefore considered. 



The meteoric theory of lunar craters is rejected, because such bodies approaching the 

 moon in any direction would make a large number of oblique impacts, their predominant angle 

 of incidence being 45°, and would therefore produce a large proportion of oval craters; but 

 the lunar craters are nearly circular; twenty-nine-thirtieths of them have less than 0.3 ellipticity. 

 The meteoric theory is therefore replaced by the moonlet theory, and this was Gdbert's original 

 and personal contribution to the problem; he set it forth with some fullness. It assumes that 

 the moon has been formed by the gradual gathering together of a vast number of small bodies, 

 or moonlots, which originally formed a ring around the earth, similar to the ring around Saturn ; 

 that successive impacts of the moonlets on the growing moon were so far separated in time 

 that, except when heated locally and temporardy by an impact, the moon was always a cold 

 planet; and that even after the moon had grown to about its present size, the surviving moon- 

 lets stdl moving in the plane of the original ring would therefore strike the moon in that plane, 

 instead of at all parts of its surface. Under such conditions, at least half the moonlets would 

 fall upon the moon with an angle of incidence of less than 30°; if the moon's attraction be 

 considered, nearly three-fourths of the impacts would have a less angle of incidence than 30°; 

 and if the effect of moonlets in influencing the moon's rotation is considered, the number of 

 nearly vertical impacts would be stdl further increased. The objection that, under this theory, 

 lunar craters should lie chiefly near the lunar equator is met by saying that as the moonlets 

 did not move precisely in the plane of the moon's equator, "their coUisions would disturb its 

 axis of rotation .... Under successive impulses thus given the moon's equator may have 

 occupied successively ail parts of its surface, without ever departing widely from the plane of 

 the moon's orbit." 



This theory is manifestly ingenious, but its acceptance is hindered by three difficulties that 

 Gilbert's essay does not overcome. One is the absence of at least a few very elliptical craters, 

 which even under the moonlet theory ought to make no insignificant fraction of the total, A 

 second difficulty is the unsystematic distribution of the circular craters; for under the moonlet 

 theory an unsystematic distribution involves the necessity of supposing that, even after the moon 

 had reached essentially its present size, the off-equator impacts of the remaining moonlets, some 

 of them to the north, others to the south, were so far from being compensatory that they tilted 

 the moon into various positions, and this in spite of the earth's action in holding the supposedly 

 prolate moon with its longer axis directed earthward. It would seem, on the contrary, that the 

 last few thousand moonlets would be unable to tilt the moon appreciably, and that they ought 

 therefore to have produced craters chiefly around an equatorial great circle, or that if they 



