THE SMITHSONIAN INSTITUTION. 163 



tliat the matter of wliich tliey are composed is as subtle 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 supposing a difference be- 

 tween shooting stars and meteoric stones, and 1 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 of their origin. 



Various theories have been devised to account for the origin of the 

 meteorites. One is that they are small planetary bodies revolving 

 around tlie sun, and at times become entangled in our atmosphere, 

 lose their orbital velocity by the resistance of the air, and fall finally 

 to the earth ; another supposes them to have been ejected from vol- 

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

 particles floating in the atmosphere. The exact nature of this last 

 theory is given by Professor C. U. Shepard, in an interesting re- 

 port on meteorites published in 1848. He* says: " The extra-terres- 

 trial origin of meteoric stones and iron masses seems likely to be more 

 and more called in question with the advance of knowledge respecting 

 such substances, and as additions continue to be made to the connected 

 sciences. Great electrical excitation is known to accompany volcanic 

 eruptions, which may reasonably be supposed to occasion some chemi- 

 cal changes in the volcanic ashes ejected ; these being wafted by the 

 ascensional force of the eruption into the regions of the magneto-polar 

 influence, may there undergo a species of magnetic analysis. The 

 most highly magnetic elements, (iron, nickel, cobalt, chromium, &c.,) 

 or compounds in which these predominate, would thereby be separated 

 and become suspended in the form of metallic dust, forming those 

 columnar clouds so often illuminated in auroral displays, and whose 

 position conforms to the direction of the dipping needle. While cer- 

 tain of the diamagnetic elements, (or combinations of them,) on the 

 otlier hand, may. under the control of the same force, be collected 

 into diflerent masses, taking up a position at right angles to the 

 former, (which Faraday has shown to be the fact in respect to such 

 bodies,) and thus produce those more or less regular arches, transverse 

 to the magnetic meridian, that are often recognised in the phenomena 

 of the aurora borealis. 



" Any great disturbance of the forces maintaining these clouds of 

 meteor-dust, like that produced by a magnetic storm, might lead to 

 the precipitation of portions of the matter thus suspended. If the 

 disturbance was confined to the magnetic dust, iron masses would fall ; 

 if to the diamagnetic dust, a non-terruginous stone ; if it should ex- 

 tend to both classes simultaneously, a blending of the two characters 

 would ensue in the precipitate, and a rain of ordinary meteoric stones 

 would take place, 



"Tlie occasional raining of meteorites might, therefore, on such a 



* I must, in justice to Professor Shepard, say that since this lecture was delivered he 

 has informed me that he no longer entertains these views ; and I would now omit the 

 criticism of them did they not exist in liis memoir imcontradicted, and also were they 

 not views still entertained by some. 



