36 On the Tails of Comets. 



most part attended) as given by many distinguished astronomers, 

 at periods very remote from each other, I am constrained to ac- 

 knowledge, high as the authority unquestionably is, that no one 

 has afforded to my mind the slightest satisfaction. Notwith- 

 standing the great number of writers on this subject and the di- 

 versity of opinions that have been promulgated, there appears to 

 have been only two prevailing theories. The more ancient of 

 these supposed the tails to be formed by the lighter parts being 

 thrown off by the resistance of the ether through which the 

 comet passed. The modern and the more generally prevailing 

 theory is, that these particles are driven off by the impulsive 

 force of the sun's rays. In each of these theories, the tails are 

 supposed to consist of matter. With regard to the former theo- 

 ry, the simple fact that the tail precedes the comet in its course 

 through a portion of its elliptical journey, is a sufhcient refuta- 

 tion ; and to afford weight or plausibility to the latter, it is neces- 

 sary to assume that the sun "blows heat and cold with the same 

 breath" — in other words, that it attracts and repels with the 

 same 7nodus operandi. If we have no evidence of a repulsive 

 force in the sun, to say nothing of a force sufficient to repel the 

 lighter particles of these bodies to a distance from the head of 

 the comet, equal to and sometimes exceeding a hundred millions 

 of miles, this theory, to say the least of it, is labored and unsat- 

 isfactory. The length of these trains is far from being exagger- 

 ated. Referring to my minutes of the late return of Halley's 

 comet, I find that, at one period, the tail, by direct vision, sub- 

 tended an angle of twenty degrees, and on some occasions, by 

 oblique vision, more than forty degrees. The tail of the comet 

 of 1689 is said to exceed sixty eight degrees, and that of the 

 comet of 16S0, ninety degrees. Making a proper allowance for 

 the faintness of the extremity of the tail, and the obstruction of 

 the view by the atmosphere of the earth, it is by no means un- 

 safe to conclude that many of them extend some hundreds of mil- 

 lions of miles from the nucleus of the comet. 



In view then of the last mentioned theory, it is by no means 

 a matter of surprise that Newton, and with him La Place and 

 Sir J. Herschel, should entertain the opinion that the more re- 

 mote particles could never be recalled by the gravitation of the 

 nucleus, and that portions of the tails were at each revolution 

 scattered in space, and hence that comets were continually 

 wastino;. 



