Examination of the Theory of a Resisting Medium. 18 
delicate experiments have hitherto failed to render such force per- 
ceptible. ‘This force shown and admitted, it will still remain to be 
demonstrated why the tail is not always situated opposite to the sun ; 
why there are sometimes several tails, making, one with another, so 
great angles ; why they form and again vanish, in so short periods of 
time; why some of them have a rapid rotary motion; and finally, 
why some comets, of which the envelope seems very light and deli- 
cate, exhibit no trace of this appendage. A crowd of other theories, 
more or less ingenious, have been proposed ; but they all equally fail 
to explain the phenomena.”’(49) 
The enormous length to which these tails have sometimes at- 
tained, has given rise to theories no less fanciful, nor yet more phi- 
losophical, respecting the consequences of such elongation. New- 
ton supposed that the extremely distant portions of these tails could 
never be recalled, by attraction, to the nucleus of the comet, but 
inust be scattered through the heavens, to be subsequently gathered 
to the different planets by attraction, and mingled with their atmos- 
pheres, to be there appropriated to supply the waste of matter spent 
upon vegetation, &c.(50) Laplace, the younger Herschel, and 
some others among the moderns, have assumed that portions of 
comets’ tails are, at each revolution, “ scattered in space,” and that, 
consequently, these bodies are continually wasting away. So in- 
definite a phrase seems not well calculated to convey any idea of 
facts; for we must suppose the matter of these tails, however elon- 
gated from the nucleus of the comets, will still obey the laws of 
gravitation to those bodies, unless brought within the stronger at- 
traction of some other body: and in either case no dissemination of 
matter would take place. But the diminution of comets from loss 
of matter, by any cause, seems not well sustained. It is true that 
Arago, in 1832, fully concurred in this view; and hence advised us 
that in the then approaching return of Halley’s comet we must not 
expect to behold so brilliant a body as the same had been at former 
periods of its return to the sun.(51) But this opinion of that as- 
tronomer he did not find supported by the actual appearance of 
Halley’s comet, in 1835; and this fact he has promptly announced. 
He has, also, collectively presented what has come down to us of 
the apparent size, length of tail, &c. of Halley’s comet at its various 
(49) Arago, Lecons d’Astronomie Professées 4 |’Observatoire Royal, p. 207—8. 
(50) Math. Prin. (vide note 12,) vol. 2, p. 371. 
(51) Tract on Comets. 
