An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

OtIS1N AND'MOTIONS o01 COMfES3. 263
that ilst'llrumenlt (the Calculus) by Jmcans of w\\hich its 0lements
had bccli ilnvestigated.
402. ]incke's comet, by its fi'equent returns, affords peculiar facilities for ascertaining tile laws of its revolution; and it
]has kept thle appointments imade for it with great exactness.
On its return ill 1839) it;cxhibited to the telescope a globlular
mtass of nebulous matter rslelbling fog, and mnoved toward its
)periltclion withl great rapidity.
IBut what has made Encke's comet plarticularly ftamous, is its
having first rcvealed to us tile existence of a Rlcsistkliqy:Jlfditill the p)lalcetary spaces. fI.t has long bleen a question whethler
t.le carth ad plainets revolve illn a perfect void, or whethler a
filuid of cxtcle rarity may not be diffllsel through space. A
)erfcct vactuumll was dccmlled most probl)ble, because no suchl
eefi'ts onl thle Imotions of the planets could be detected as indicated tlhat they encountcired a resistingw medium. ltBut a feather
or a lock of cotton, propelled with great velocity, miglt render
obvious te resistanlice of a mediumn whlich would not be perceptible inl tlhe motions of (a cannol-ball. Accordingly, Elicke's
collect is tlhougllt to hlave plainly suffleed a retardation fitom
encountering a resistillg medium in the pllaletary regions.
IThe effect of this resistance, fiom the first discovery of tlle
colmet to tile lprcscent tiltme, has been to diminish tthe timec of its
revolution about two days. Such a resistanlC ), by destroy)illng
part of thle projectile force, wouldl cause tmhe comet to approach
nIcarr' to tlhe sun, ald tIhus to lhatv its periodic time shortcle(l.
Thc ulltimlate effect of thlis cause ill be to brinog the comet
nearer to the sun at every revolution, until it finally fialls ilto
that luminary, althougl many thousll and years will be required
to p)todultce thlis catastrophec.* It is conceivable, indeeld, that
the efiects of such a resistance may be counteracted by the attraction of one or more of the planets lnear whicht it may pas)ss
in its successive returns to the sul. lt is peculiarly intceesting
to see a portion of matter of a tenuity exceedilng the thinnest
fog, pursuil.ng its l)atll in space, in obedietce to. the sante laws
as t!hose whtich regulate sucth large an1d heavy bodies as Jupliter
* Ifally'ys comet, at its Ictturn in 1835, did inot appear to be: afftcted by the
supposed vesisting medium, and its existence is considered as still doiubtful.