SKCT. XXXVI. COMETS. 337 



magnetism can no longer be regarded as a separate in- 

 dependent principle. Although the evolution of light 

 and heat during the passage of the electric fluid may be 

 from the compression of the air, yet the development 

 of electricity by heat, the influence of heat on magnetic 

 bodies, and that of light on the vibration of the compass, 

 show an occult connection between all these agents, 

 which probably will one day be revealed. In the mean 

 time it opens a noble field of experimental research to 

 philosophers of the present, perhaps of future ages. 



SECTION XXXVI. 



Ethereal Medium Comets Do not disturb the Solar System Their 

 Orbits and Disturbances M. Faye's Comet, probably the same with 

 Level's Periods of other three known Halley's Acceleration in the 

 Mean Motions of Encke's and Biela's Comets The Shock of a Comet- 

 Disturbing Action of the Earth and Planets on Encke's and Biela's 

 Comets Velocity of Comets The Great Comet of 1843 Physical Con- 

 stitution Shine by borrowed Light Estimation of their Number. 



IN considering the constitution of the earth and the 

 fluids which surround it, various subjects have presented 

 themselves to our notice, of which some, for aught we 

 know, are confined to the planet we inhabit ; some are 

 common to it and to the other bodies of our system. 

 But an all-pervading ether probably fills the whole visi- 

 ble creation, and conveys, in the form of light, tremors 

 which may have been excited in the deepest recesses 

 of the universe thousands of years before we were called 

 into being. The existence of such a medium, though 

 at first hypothetical, is nearly proved by the undulatory 

 theory of light, and rendered all but certain within a 

 few years by the motion of comets, and by its action 

 upon the vapors of which they are chiefly composed. 

 It has often been imagined, that, in addition to the ef- 

 fects of heat and electricity, the tails of comets have 

 infused new substances into our atmosphere. Possibly 

 the earth may attract some of that nebulous matter, 

 since the vapors raised by the sun's heat, when the 

 comets are in perihelio, and which form their tails, are 

 scattered through space in their passage to their aphe- 

 lion ; but it has hitherto produced no effect, nor have 

 22 FF 



