MEMOIR ON METEORITES. 295 



their orbit be around the sun and their velocity sixteen miles a 

 second — there is one thing we know; namely, that these bodies 

 do enter our atmosphere, and, it is but right to assume, often 

 pass through the atmosphere without falling to the earth, 

 sometimes passing through the very uppermost portion of that 

 medium, at other times lower. What becomes of their original 

 assumed velocity after this passage? As they can be so checked 

 as to be drawn to the earth's surface, and thus stopped alto- 

 gether in their passage, their velocities may be changed to any 

 velocity from sixteen miles a second to zero, according to the 

 amount of resistance they meet with ; and what is equally true 

 in this connection is that when the velocity falls below six miles 

 a second (or thereabout) they can no longer escape from the 

 attraction of the earth and resume their solar orbit, but must 

 revolve as a satellite around the earth until ultimately brought 

 to its surface by repeated disturbances. 



The deduction from the above fact is as follows : that as the 

 most correct observations have never given a velocity of less 

 than nine miles a second to a shooting-star, it is reasonable to 

 suppose that none have ever entered our atmosphere, or, what 

 is perhaps still more reasonable, that the matter of which they 

 are- composed is as subtile as that of Encke's comet, and any 

 contact with even the uppermost limit of the atmosphere 

 destroys their velocity and disperses the matter of which they 

 are composed. Other grounds might be mentioned for sup- 

 posing a difference between shooting-stars and meteoric stones, 

 and I have dwelt on it thus much because it is conceived of 

 prime importance in pursuing the correct path that is to lead 

 to the discovery (if it can be made) of their origin. It is also 

 of no small value to the beautiful and probable theory of shoot- 

 ing-stars that we should separate every thing from it that may 

 tend to affect its plausibility. 



Various theories have been devised to account for their 

 origin. One is that they are small planetary bodies revolving 

 around the sun, and that at times they become entangled in 

 the atmosphere, lose their orbital velocity by the resistance 

 of the atmosphere, and are finally attracted to the earth. They 

 are also supposed to have been ejected from the volcanoes of 

 the moon; and lastly, they are considered as formed from 

 particles floating in the atmosphere. The exact nature of this 



