14. Examination of the Theory of a Resisting Medium. 
former apparitions ; and contrasted this with the results of the care- 
ful and accurate observations upon the same body, made at various 
points, during its last appearance. At the close of these he adds: 
“‘TIf the reader will take the trouble to compare what I record of 
the comet of 1835 with the circumstances of its former apparitions, 
he certainly will not find in this collection of phenomena, the proof 
that Halley’s comet is gradually diminishing. I will even say that if, 
in a matter so delicate, observations made at very different periods of 
the year, will authorize any positive deduction, that which would most 
distinctly result from the two passages of 1759 and 1835, would be 
that the comet had increased in size during that interval. I ought 
to seize, with the more eagerness, this occasion to combat an errour 
extensively accredited, (a belief in the constant wasting away of com- 
ets,) because I believe | have somewhat contributed to its dissemina- 
tion.”(52) This review of the theory of the diminution of comets, 
otherwise foreign to our subject, seemed demanded by the assumption 
of some that matter thus lost from these bodies will remain diffused 
through the celestial regions, of course offering constant obstruction 
to the progressive motion of the planets and comets. How such 
matter is to be maintained in this state of diffusion, has not, so far 
as we know, been explained; nor is it easy for us to conceive how - 
the body resisted or encountered by it shall be prevented from ap- 
propriating it to itself, by adding it to its own mass.(53) 
Comets, from their great volumes, as compared with their masses, 
have justly been considered, of all celestial bodies, the most neces- 
sarily subject to the action of any resisting medium there may be in 
the regions in which they are moved. They are known to be sub- 
ject to great disturbances, in their orbits, by the attraction of the 
(52) Arago, Sur la derniére apparition de la cométe de Halley; Annuaire, 
pour Van 1836. 
(53) The following “ poetical license” occurs in the younger Herschel’s Treat- 
ise on Astronomy, a late work, now used in some of the schools of this country. 
It contrasts very strangely with the really sane and valuable portions of that work, 
and it would hardly be supposed possible that it is from the same pen with these. 
The author is treating of Zodiacal light, upon which he thus fancifully expresses 
himself. ‘It is manifestly in the nature of a thin, lenticularly-formed at- 
mosphere, surrounding the sun, and extending at least beyond the orbit of Mer- 
eury and even Venus, and may be conjectured to be no other than the denser 
parts of that medium, which, as we have reason to believe, resists the motion of 
comets; loaded, perhaps, with the actual materials of the tails of millions of those 
bodies, of which they have been stripped, in their successive perihelion passages, 
and which may be slowly subsiding into the sun”! 
