216 METKOIIOLOGY. 



1 1 will only require us to look at this for one moment, and in one point 

 of view. Inherently, comets possess neither light nor heat ; that 

 they are visible by virtue of light derived from the sun, has been 

 proved by a method pointed out some years ago by M. Arago. He 

 has shewn that if the light of comets were proper to themselves they 

 would continue visible so long as they possessed any sensible mag- 

 nitude, and would not disappear until their distance was so great as 

 to reduce their apparent bulk below any appreciable value ; whereas, 

 they become invisible even whilst they have a sensible magnitude, and 

 some time before their distance alone would render them invisible ; 

 (the argument of M. Arago would be too long to introduce here 

 more fully). The light, then, which we receive from comets is no 

 more calorofic than that which is reflected by the moon, and is, at 

 the same time, incomparably more feeble. Whoever reflects that 

 the light received from comets is of the same kind as that reflected 

 from the moon, will at once perceive, when it is known that the 

 brilliant light of the moon does not sensibly affect the thermometer, 

 the impossibility of any influence being exerted by the feeble and 

 diffused glimmerings from a vapoury and far distant comet. If it 

 be supposed that the influence be dependant on its quantity of 

 matter, rather than its light or heat, the absurdity is equally palpable, 

 for so attenuated are comets, that in the immediate neighbourhood 

 of the nucleus small stars are distinctly visible through the vapouiy 

 mass ; and, besides, if comets had really a power over meteorological 

 phenomena, when we regard the number kno^^ii to belong to the 

 system, some of them of very short period, and the gi'eat probability 

 of many more undiscovered, being circulating round us, we cannot 

 select a moment in which the earth may be supposed to be free 

 from such agency, and, of course, uncertainty must characterise at 

 all times, on his own principles, the calculations of the astrologer. 



It will have been seen that what has been advanced in these 

 pages, beyond the attempt to expose en'ors, which materially 

 damage true philosophy, is intended rather for the purpose of stimu- 

 lating enquiry, than of explaining a mysterious and involved 

 science ; for we see from the many physical causes known to be 

 effective in the earth's atmosphere, as well locally as generally, that 

 it is not surprising that meteorology should be beset with unsolved 

 diflficulties. Nevertheless we may regard it as certain, that although 



