On Lunar ProjeetileSy Aeroliths, and Notices of Comets. 203 



changes by the effects of frost and putrefaction, no saccharine 

 matter is perceptible, nor nitre to be found : From whence come 

 hey ? 1 remain, dear sir, yours, 



Feb. 6, 1818. Gavin Inglis. 



XXXV. On supposed Lunar Projectiles, on Aeroliths ; and on 



Notices of Comets, 



To Mr. Tilloch. 



Sir, — A doubt you have bestowed more elegance and delicacy 

 oi engraving on my hasty illustrative d/agraui than it merited. 



I anticipate some objections. 



First, That the course of a projectile from the moon would be 

 much nearar rectilineal. 



But in this particular instance it would require to be much 

 otherwise to allow for any possihility of reaching the earth at 

 all had it been projected from the moon; then not far distant from 

 the meridian of our antipodes. 



Had it been thrown from the moon it is pretty evident, from 

 the path described, that we saw it while near its utmost altitude 

 above our hemisphere. 



Had its course been nearly rectilineal, its apparent path must 

 hate differed greatly from that described. 



It follows that it was either a volcaiiic projection from the 

 earth, ox inflamed hydrogen, in our atmosphere,'or an electriciyxQ- 

 ball, or a proper aemlithic* mass generated in the air in the man- 

 ner suggested in the communication which introduces mine. 



The advocates for lunar projectiles will perhaps say, that to 

 carry such a projectile to the earth it needs to proceed no farther 

 than to the point where, mass compared with distance, the ter- 

 restrial attraction would overbalance the lunai-,or about 7 or 8000 

 miles from the moon's centre. 



But I own my doubt, whether it would be so far within the 

 earth's attraction as to fall on it unless the projectile force first 

 carried it to a distance within the common centre oi gravity of 

 the moon and earth, or about 6000 miles from the earth's centre. 



For it is to be considered, that to carry a planet, primary or 

 secondary, from its orbit, requires not merely a force greatest at 

 the point of its immediate influence than the attraction of the 

 primary, but so much greater as will be incompatible, when it has 

 passed that point, with its being at all retained in its orlit. 

 Now this demands a far more preponderating attraction than is 

 required to alter considerably for the time the figure oi the orbit. 

 A comet, for instEmce, in its aphelion may be much more attracted 

 by Jupiter or Saturn or the Hersilian planet, in the moment of 



passing 



