Remarks on the Tails of Comets. 63 



ter to a hemispherical cap,) and the zone between the nucleus 

 and the envelope in the great comet of 1811, exceeded at one 

 time 27,000 miles. And finally, while the analogy of comets to 

 planets is acknowledged in reference to their orbital motion, it 

 is evident that the general phenomena of comets are quite dissim- 

 ilar. Describing, for the most part, orbits of great eccentricity, 

 some of them moving nearly at right angles with the ecliptic, and 

 not a few directly contrary to the order of the signs of the zodiac, 

 the assumption of a chemical discrepancy between the envelope 

 of a comet and planetary atmosphere, would not be an unreason- 

 able one, and a very slight change in the constituents of the earth's 

 atmosphere, it it is well known, would greatly affect its refractive 

 properties. According to the experiments of Biot, hydrogen gas 

 has six times the refractive power of atmospheric air. In view 

 of these circumstances, the fact that the planets have no tails, 

 and that some comets have not this appendage, presents no objec- 

 tion to the theory which I have ventured to suggest. 



The final objection offered by the writer, (though others he 

 thinks might be adduced,) is the plurality of tails of the comet of 

 1744, and the secondary tail of the comet of 1823, and that of 

 1835. In reference to the comet of 1744, I have no doubt of its 

 plurality of tails, though great allowance is to be made for exag- 

 gerated description, as well as optical illusion. The great comet 

 of 1811 was at one time so situated relatively to the earth and 

 sun, that the tail had somewhat the appearance of a fan, and this 

 was the distinguishing feature of the comet of 1744, and the tes- 

 timony is worthy of credence, that there were several dark zones 

 diverging from the envelope to the extremity of the tail, giving 

 to the whole an appearance of a plurality or an assemblage of 

 tails. The comet of 1769 was also accompanied with a plurali- 

 ty of tails, a particular description of which is given by Mes- 

 sier. To reconcile this with my theory, is only to assume that 

 the envelope is not perfectly homogeneous, an assumption abund- 

 antly confirmed by observation. The account given by Scroeter 

 of the comet of 1799 and 1807, of occasional obscurity in the 

 head of the comet, is a striking circumstance in proof of cloudy 

 regions, and this is the explanation given by that writer. But I 

 do not rely on these facts only for an explanation of the eccen- 

 tricities of the comet of 1744. There are other and more direct 

 causes of the phenomena. This comet at its perihelion approach- 



