202 WARREN UPHAM, M.A., F.G.8.A., ON THE NEBULAR AND 



Survey.* This very remarkable and ingenious explanation 

 seems largely identical with the later planetesimal hypothesis 

 of Chamberlin, so far as that hypothesis deals with the 

 segregation of the originally nebulous matter to form planets 

 and satellites. Mr. Gilbert writes : 



" . . . It is my hypothesis that before our moon came into 

 existence the earth was surrounded by a ring similar to the Satur- 

 nian ring ; that the small bodies constituting this ring afterward 

 gradually coalesced, gathering first around a large number of nuclei, 

 and finally all uniting in a single sphere, the moon. Under this 

 hypothesis the lunar craters are the scars produced by the collision 

 of those minor aggregations, or moonlets, which last surrendered 

 their individuality. 



" . . . The introduction of the hypothesis of a Saturnian 

 ring thus accomplishes much toward the reconciliation of the impact 

 theory with the circular outline of the lunar craters. . . . 



" In fine, the hypothesis of the Saturnian ring, by restricting the 

 colliding bodies to a single plane, by substituting a low initial 

 velocity and thus rendering the moon's attraction the dominant 

 influence, and by introducing a system of directions controlling, and 

 therefore adjusted to, the moon's rotation, relieves the meteoric 

 theory of its most formidable difficulty. It also explains in a 

 simple way the abundance of colliding bodies of a different order of 

 magnitude from ordinary meteorites and aerolites. . . . 



" The velocity of impact, depending chiefly on the moon's attrac- 

 tion, 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 



* " The Moon's Face, a Study of the Origin of its Features," address 

 as retiring President, delivered December 10, 1892, Bulletin of the 

 Philosophical Society of Washington, D.C., vol. xii, pp. 241-292, with one 

 plate and 14 figures in the text ; published April, 1893. 



Referring to early suggestions of meteoric accumulation of the moon, 

 and of other cosmic bodies, Mr. Gilbert said in this paper (1892) : "I 

 have discovered no published statement of meteoric theories more than 

 twenty years old, but the idea is older and various obscure allusions 

 indicate that it was earlier in print. Proctor makes a meteoric 

 suggestion in 1873 (The Moon, p. 346), and advocates it in 1878 

 (Belgravia, vol. xxxvi, p. 153). A meteoric theory is said to be contained 

 in Die Physiognomie des Mondes, by ' Asterios, 5 Nordlingen, 1879. 

 A. Meydenbauer advances another in ' Sirius,' for February, 1882." 



With these publications, compare The Meteoritic Hypothesis, 1890, by 

 Lockyer, before cited, and a most important paper by Prof. George H. 

 Darwin, "On the Mechanical Conditions of Swarms of Meteorites and 

 on Theories of Cosmogony," Phil. Trans. Royal Society, 1888. 



