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 ke 
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 ef its elliptical journey, is a sufficient 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 modus 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 oid to exceed sixty eight degrees, and that of the 
comet of 1680, 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 — 
wasting. 
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