358 COMETS. SECT. XXXV. 



SECTION XXXV. 



Ethereal Medium Comets Do not disturb the Solar System Their 

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

 Lexel's Periods of other three known Acceleration in the mean 

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

 turbing Action of the Earth and Planets on Encke's and Biela's Comets 

 Velocity of Comets The Comet of 1264 The great Comet of 

 1343 Physical Constitution Shine by borrowed Light Estima- 

 tion 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 must fill the whole 

 visible creation, since it 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 

 proved by the undulatory theory of light, and rendered certain 

 by the motion of comets, and by its action upon the vapours of 

 which they are chiefly composed. It has often been imagined 

 that 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 thermometer of a structure so delicate 

 that it would have made the hundredth part of a degree evident. 

 In all probability, the tails of comets may have passed over the 

 earth without its inhabitants being conscious of their presence ; 

 and there is reason to believe that the tail of the great comet of 

 1843 did so. M. Valz observed that the light of a brilliant 



