38 On the l^ails of Comets. 



the proportion of the square of the diminution of distance, — 

 again that the tail should have a cylindrical and hollow appear- 

 ance, the rays of light being at least partially obstructed by the nu- 

 cleus ; moreover, that the tail should be curved, by the necessary 

 effect of aberration. I apprehend it will be acknowledged that the 

 weight of testimony is decidedly favorable to the fact that the nu- 

 clei of comets, though they generally resemble planets in form and 

 brilliancy, may not be solid or opaque, inasmuch as some are un- 

 questionably transparent, and the quantity of matter in all is ex- 

 ceedingly inconsiderable. 



Professor Struve saw a star of the eleventh magnitude through 

 the Encke comet ; Sir William Herschel noticed one of the 

 sixth magnitude through the centre of the comet of 1795; and 

 his illustrious son, in a memoir communicated to the Royal As- 

 tronomical Society, mentions that he saw a cluster of stars of the 

 sixteenth magnitude very near the centre of Biela's comet. Not- 

 withstanding this tenuity, an increased density may always be 

 noticed toward the centre of the head, except in a few small 

 comets unaccompanied with trains. 



Astronomers of all ages seem to have been inclined to a belief 

 in an ethereal medium, and the present one has atforded a conclu- 

 sive evidence of its existence, in its effect upon the duration of 

 the revolution of the Encke comet. Professor Encke in a dis- 

 sertation on this subject, after giving the minutias of his observa- 

 tions, very modestly remarks — " If I may be permitted to express 

 my opinion on a subject which for twelve years has incessantly 

 occupied me, in treating which I have avoided no method, how- 

 ever circuitous, no kind of verification, in order to reach the truth 

 so far as it lay in my power ; I can not consider it otherwise than 

 completely established, that an extraordinary connection is neces- 

 sary for Pon's* comet, and equally certain that the principal part 

 of it consists in an increase of the mean motion proportionate to 

 the time." Professor Airy, in an appendix to a translation of 

 Encke's memoir, adds — " I can not but express my belief that 

 the principal point of the theory, namely, an effect exactly sim- 

 ilar to that which a resisting medium would produce, is perfectly 

 established by the reasoning of Professor Encke." Arago, in 

 speaking of the discrepancy between the result of calculation and 



'•* Called by others Encke's comet. 



