62 Remarks on the Tails of Comets. 



be astonishing. If the tail of a comet were a brilliant object, we 

 might expect to witness from its radiance, at least a degree of the 

 light imparted by the moon, and by the planets, it having frequent- 

 ly the magnitude of many moons and planets ; but no such effect is 

 witnessed. Yet in those regions far beyond the atmosphere of 

 the earth, a vast combination of faintly illuminated particles, it is 

 rational to conclude, would be distinctly visible, even though 

 an individual point might be beyond telescopic power. 



"Another objection," says the writer, " to this theory is, that if 

 the rays of the sun are refracted by the vapor of the comets, so 

 as to form a luminous train, the same thing should occur to the 

 planets, at least to the two inferior planets." So many objections 

 at once present themselves to this view, that it has occurred to me 

 to give the summary one, that som,e comets have no tails. Like 

 causes, it is to be admitted, should produce like effects ; but not 

 under unlike circumstances. I have not attempted to explain 

 why some comets have no tails, a subject far more difficult than 

 the one proposed. In the first place, why did the writer say, "at 

 least, the two inferior planets." It is evident that one at least 

 of the many manifest points of difference between the circum- 

 stances of a planet and those of a comet had presented itself to 

 his mind. That the planets, in common with the earth, are ac- 

 companied by atmospheres, I have no doubt. That of the earth 

 is exceedingly limited, — that of the moon still more so ; nor is 

 there conclusive testimony that the atmosphere of any planet 

 bears any considerable proportion to its diameter. The atmos- 

 phere of the earth capable of reflecting the sun's light, does not 

 exceed the one hundred and sixtieth part of the earth's radii, and 

 a portion of this is sufficiently dense to absorb a measure of the 

 sun's light, and the want of combination in the few more vivid 

 rays which escape material absorption, even assuming that the 

 chemical properties of the atmosphere are identical with those of 

 a comet's envelope, would render them invisible. In the second 

 place, though comets in all cases are accompanied with a shining 

 envelope, in appearance analogous to an atmosphere, yet its rela- 

 tive position bears no resemblance whatever to the atmosphere of 

 the earth, nor to those phenomena which indicate the existence of 

 atmospheres in the planets. The envelope of a comet which has 

 a tail, is visible only on the side of the comet next to the sun, 

 and detached entirely from the nucleus, (compared by some wri- 



