SECT, xxxvi.] OF COMETS. 385 



SECTION XXXVI. 



Ethereal Medium Comets Do not disturb the Solar System Their Orbits 

 and Disturbances M. Faye's Comet probably the same with Lexers- 

 Periods of other three known 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 1343 Physical Constitution Shine by bor- 

 rowed Light Estimation of their Number. 



IN considering the constitution of the earth, and the fluids 

 which surround it, various subjects have presented them- 

 selves to our notice, of which some, for aught we know, 

 are confined to the planet we inhabit ; some are common to 

 it a*nd to the other bodies of our system. But an all- 

 pervading ether probably fills the whole visible 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 vapours of which they 

 are chiefly composed. It has often been imagined that, in 

 addition to the effects 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 vapours 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 aphelion ; 

 but it has hitherto produced no effect, nor have the seasons 

 ever been influenced by these bodies. The light of the 

 comet of the year 1811, which was so brilliant, did not 

 impart any heat even when condensed on the bulb of a 



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