ON COMETS. 207 



jects differ extremely in magnitude, connecting it with the ouratmos- 

 luminous phenomena observed in our atmosphere, such as p 

 the aurora borealis, luminous clouds, and many other lu- 

 minous meteors. All these are so many hiciferous vapours, 

 though of different kinds ; which, being raised in the at- 

 mosphere iu their compound state, and meeting with some- 

 thing to decompofe them, emit the light, that entered into 

 their composition. 



To determine with more precision what relates to the Proof that th* 



Sun, we should consider what led Dr. Herschel to the dis- Ijf ht . rf the 



Sun is not 

 covery, that its light did not issue directly from its solid f r0 m its body. 



body, but from an atmosphere surrounding it. This was 

 its changeable spots; a phenomenon, that had greatly per- 

 plexed astronomers and natural philosophers, but which is 

 clearly explained by the discovery of Dr. Herschel, that 

 these spots are the body of the Sun itself, nonluminous, and 

 seen through transparent parts of its luminous envelope. 

 This shows, that, though the light issues from a fluid be- 

 longing to that kind of atmosphere, which surrounds the 

 Sun, this fluid is not the atmosphere itself, but is simply 

 mixed with it ; and this not completely throughout, since 

 there are parts that remain transparent, or from which no 

 light emanates. 



Another consideration determines this phenomena still }g u t it is frona: 

 more precisely. The disk of the Sun, as measured by us, a P er ™ a nent 

 is properly that of its lucid covering, in the edge of which ** 



Dr. Herschel has discerned gaps, when any large spot ar- 

 rives there. This covering then has a constantly uniform 

 thickness, since we perceive no change in the diameter of the 

 Sun, which is determined by it. Now this is not the cha- not of the 

 racter of an atmosphere, which in the general acceptation »«tureofan 

 of the term, taken from the atmosphere of the Earth, - g a mos P ,ere » 

 considered as formed of one or several expansible fluids, the 

 density of which diminishes in proportion as the strata are / 



more remote from their base, so that they extend by imper- 

 ceptible gradations to an indefinite distance. In fact, when 

 astronomers have spoken of an atmosphere of the Sun, they 

 have ascribed to it a vast extent. Thus, if the luciferous 

 fluid, that surrounds the Sun, have such a constant thick- 

 ness, and be so well defined, that it has hitherto been taken 



for 



