AuGi'ST 1, 1893.] 



KNOWLEDGE 



151 



tudinous craters near the southern part of the moon are 

 strongly suggestive of the kind of process I have referred 

 to, and that, in fact, if one judged solely by appearances, 

 one would be disposed to adopt somewhat confidently the 

 theory that the moon had had her present surface contour 

 chiefly formed by meteoric downfalls during the period of 

 her existence when she was plastic to impressions from 

 without. I 

 am, however, 

 sensible that 

 the great cra- 

 ters, under 

 close tele- 

 scopic scru- 

 tiny, by no 

 means corre- 

 spond in 

 appearance 

 to what we 

 should expect 

 if theij were 

 formed by the 

 downfall of 

 great masses 

 from without. 

 The regular, 

 and we may 

 almost say 

 battlemented, 

 aspect of some 

 of these cra- 

 ters, the level 

 floor, and the 

 central peaks 

 so commonly 

 recognized, 

 seem alto- 

 gether differ- 

 ent from what 

 we should 

 expect if a 

 mass fell from 

 outer space 



upon 



moon's 

 face, 

 indeed , 

 possible 



the 



sur- 



It is, 



just 



that 



under the tre- 

 mendous heat 

 generated by 

 the downfall, 

 a vast circular 

 region of the 

 moon'ssurface 

 would be ren- 

 dered liquid, 

 and that in 

 rapidly solidi- 

 fying while 

 still traversed 

 downfall, 

 result.' 



Prom a jihotograph of the Moon taken by th 



by the ring waves resulting from the 

 something like the present condition would 



More^recently the meteoric theory of the formation of 

 lunar craters has been taken up and considerably elaborated 

 by an American, Mr. G. E. Gilbert, who has made the theory 

 the subject of an address, delivered when retiring from the 

 presidency of the Washington Philosophical Society, on 

 December 10th, 1892. Recognizing the difficulty alluded 



to by Mr. Proctor, viz., that most of the lunar craters are 

 circular, while if the meteoric bodies came from outer 

 space many of them ought to strike the moon's surface 

 very obhquely and produce elliptic rings, Mr. Gilbert made 

 a series of experiments in the laboratory, and found that 



nl« «L^'"°-'?' 1 ''''■' ^^r^l °l'liq"ely against a target of 

 plastic materials a crater-shaped hole of elliptic contour 



was formed. 

 In order to ob- 

 viate this ob- 

 jection to the 

 theory, he as- 

 sumes that the 

 bombarding 

 masses which 

 gave rise to 

 the lunar cra- 

 ters did not 

 come from 

 outer space, 

 but were origi- 

 nally parts of 

 a ring about 

 the earth, 

 similar to the 

 ring which 

 encircles the 

 planet Saturn. 

 From this ring 

 he supposes 

 that the moon 

 was gradually 

 formed, the 

 small bodies 

 constituting 

 the ring hav- 

 ing first coa- 

 lesced into a 

 large number 

 of moonlets, 

 which finally 

 all united into 

 a single 

 sphere. Ac- 

 cording to this 

 hypothesis the 

 lunar craters 

 are the scars 

 produced by 

 the collision of 

 the moonlets 

 which last sur- 

 rendered their 

 individuality ; 

 and, according 

 to Mr. Gilbert 

 and a mathe- 

 matical friend 



e Brotliers Henry on the 29th May, 1S90. '^^^ aided him 



in the investi- 

 gation, 58 per cent, of the moonlets would under the 

 circumstances imagined strike the surfax;e of the 

 moon, making an angle of less than 20° with the 

 vertical, while 70 per cent, would strike at an angle 

 of less than 30°, and 80 per cent, at an angle of less 

 than 40°. From laboratory experiments, Mr. Gilbert 

 found that the ellipticity of the scars on his plastic target 

 increased slowly up to an incidence of 40° to the 

 vertical, and that beyond that incidence the resulting scars 



