202 OBSERVATIONS ON ERUPTIONS OF LUNAR VOLCANOS. 



More particular j n fig. 4, p] ate x. fuppofing E and M to be the centers 

 the probability °^ l ^ e ea rth an d moon ;PQK to be a path which a body muft 

 of projeftiles defcribe, fo as to be equally attracted by both ; it will appear 

 Mrth 8 tot£ thC evident from a ver y ea fy calculation, that, P Q K is a circle 

 moon. vvhofe radius C P is equal to about 24£ femidiameters of the 



moon, and the diftance of whofe center C, from the center 

 of the moon, is equal 2 T femidiamelers (nearly).' If there- 

 fore, the lunar volcanos in any part of the hemifphere of that 

 planet, which is vifible to us, mould project bodies with a 

 force fufficient to carry them through 24 of her femidiameter$ ; 

 i. e. with a velocity of about * 7000 feet in a fecond, they 

 muft neceflarily throw them within the fphere of the earth's at- 

 traction. It may be faid, however, that the atmofphere of 

 the moon, although rarer and of lefs altitude than that of the 

 earth, may yet be con liderable enough to afford fome refiftance 

 to the motion of bodies ; allowing, therefore, (the utmoft 

 that can be allowed) that a body projected from a lunar 

 volcano, has a refiftance equivalent to that of two miles of an 

 atmofphere of equal denfity with ours, and fuppofing the 

 velocity of projection to be 12000 feet per fecond, and the 

 body to be a fphere whofe diameter is one foot, and fpecific 

 gravity 10,000 times that of the atmofphere j it would lofe 

 in its paffage lefs than T of its firft velocity, and would ftill 

 retain more than fufficient force to carry it within the fphere 

 Terreftrial vol- f tne earth's attraaion. That the volcanos of the earth 

 duce^the re- throw out heavy bodies with a force at leaft equal to this, 

 ^uifite velocity, feems eafily proved; for a body fuch as we have fuppofed, 

 if proje&ed upwards with the above-mentioned velocity, 

 would rife to the height of about nine miles from the earth's 

 furface, in a medium of equal refiftance with our atmofphere. 

 It may be objected, that the atmofphere becomes confiderably 

 rarer at that height, fo as to render the calculation of the 

 effect- of refiftance greater than it would be in reality ; this, 

 however, may be more than counterbalanced by the effect of 

 the elafticity of the atmofphere, and of the vacuum produced 

 by bodies moving with fuch very great velocity. That volcanos 

 frequently during an eruption, throw up heavy bodies to the 

 height of eight or nine miles, will, I believe, be acknow- 



* It was by rniftake that I ftated in my laft letter, the velocity 

 ceceflary at 12,000 feet per fecond. 



ledged. 



