Mr. R. P. Greg on the Lunar Origin of Aerolites. 431 



" The various asti'onomers cited concur in the opinion, that 

 a body projected from the moon with the velocity of about 

 8000 feet per second, would go beyond the mutual point of 

 attraction between the earth and moon, and already having an 

 orbital velocity, may become a satellite of the earth with a modi- 

 fied orbit. 



" The important question then for consideration is, the force 

 requisite to produce this velocity. The force exercised in terres- 

 trial volcanoes varies. According to Dr. Peters, who made ob- 

 servations on ^Etna, the velocity of some of the stones was 1250 

 feet a second, and observations made on the peak of TenerifFe 

 gave 3000 feet a second. Assuming, however, the former velo- 

 city to be the maximum of terrestrial volcanic effects, the velocity 

 with which the bodies started (stones with a specific gravity of 

 about 3'00) must have exceeded 2000 feet a second to permit of 

 an absorbed velocity of 1250 feet through the denser portions of 

 our atmosphere. Now suppose the force of the extinct volcanoes 

 of the moon to have equalled that of Ji]tna, the foi'ce would have 

 been more than sufficient to have projected masses of matter at 

 a velocity exceeding 8000 feet a second; for the resistance to be 

 overcome by the projectile force, is the attractive force of the 

 moon, which is from tive to six times less than that of the earth, 

 so that the same projectile force in the two bodies would produce 

 vastly gi-eater velocities on the moon than on the earth, discard- 

 ing of course atmospheric resistance, of which there is none in 

 the moon*/' 



In the following, I think Dr. Smith, as he does in several of 

 his arguments, rather begs the question in his anxiety to make 

 the moon's position and physical powers favour as much as pos- 

 sible the production and discharge of aerolites to the earth. It 

 would appear to me more reasonable and consistent with our 

 ideas of terrestrial volcanoes to suppose, that the total absence 

 of water and atmosphere in the moon is favourable to the notion, 

 that volcanic action there is of a less violent and explosive cha- 

 racter than on the earth. Dr. Smith says, however, — 



" But doubtless, were the truth of the matter known, the 

 projectile force of lunar volcanoes far exceeded that of any tcr- 

 I'cstrial volcanoes extinct or recent, and this we infer from the 

 enormous craters of elevation to be seen upon its surface, and 

 their great elevation above the general surface of the moon, with 

 their borders thousands of feet above their centre ; all of which 



• The editors of the American Journal here insert the following in a 

 foot-note: — 



" It would require at the moon the same force to produce an initial ve- 

 locitj' of 8()0() feet a second ;is at tlic earth ; and the difference of rate at 

 the end of the first second would be slight (discarding from consideration 

 the atmosphere). — Eds." 



