THE GREAT LUNAR CRATER TVCHO. 91 



sition that tliey were due to meteoric rain falliug- wlieu the inooii was 

 in a plastic conditiou. Indeed, it is somewhat remarkable how strik- 

 ingly certain parts of the moon resemble a surface which has been 

 rained upon, while sufficiently plastic to receive the impressions, but 

 not too soft to retain them. N'or is it any valid ol)jection to this sup- 

 position, that the rings left by meteoric downfall would only be circu- 

 lar when the falling matter chanced to strike the moon's surface 

 squarely, for it is far more probable that even when the surface was 

 struck obliquely, and the opening first formed by the meteoric mass or 

 cloud of bodies was therefore markedly elliptic, the plastic surface 

 would close in round the place of impact until the impression actually 

 formed had assumed a nearly circular shape." After inviting- attention 

 to the lunar photographs published with his book, Mr. Proctor contin- 

 ues: " It will be seen that the multitudinous 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 ccnitour 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 

 craters, under close telescopic scrutiny, by no means corresjiond in 

 appearance to what we should expect if they were formed by the down- 

 fall of great masses from without. The regular, and we may almost 

 say battlemented, aspect of some of these craters, the level floor, and 

 the central peaks so commonly recognized, seem altogether different 

 from what we should expect if a mass fell from outer space upon the 

 moon's surface. It is indeed just possible that under the tremendous 

 heat generated by the downfall, a vast circular region of tlie moon's 

 surface would be rendered liquid, and that in rai^idly solidifying while 

 still traversed by the ring waves resulting from the downfall, something 

 like the iiresent condition would result." 



More recently the meteoric theory of the formation of lunar craters 

 has been taken up and considerably elaborated by an American, Mr. 

 G. K. Gilbert, who has made the theory the subject of an address deliv- 

 ered when retiring from the presidency of the Washington Philosophical 

 Society on December 10, 1802. Kecognizing the difficulty alluded to by 

 Mr. Proct<n', viz, that most of the lunar craters are circular, Avhile if the 

 meteoric bodies came from outer space many of them ought to strike 

 the moon's surface very obliquely and produce elliptic rings, Mr. (xil- 

 bert made a series of experinicnts in the laboratory and found that 

 when projectiles were thrown obliquely against a target of plastic mate- 

 rials a crater-shaped hole of elliptic contour was formed. In order to 

 obviate this objection to the theory he assumes that the bombarding 

 masses which gave rise to the lunar craters did not come from outer 

 space, but were originally parts of a ring- about the earth similar to the 

 ring which encircles the planet Saturn. From this ring- he supposes 



