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

.CoMLtN;'s.I 253
periods of revolution, whichlsubtjct ttlem ftrequently to tht
view of astronlomcrs. I1iela's comet, atits return ill 1816 displayed anothler remarkable feature...a separantion. ianto thwo (b'tie2t parts. T'lhis str ang e peculiarity was first seemi frolm the.Observatory of Yale College, by, Mecssrs. Ilerrick and B1radllcy,
but was first publicly alnnounced from: tflhe Observatory at'Waslllgton. At olne, tieb, the distance of one nucleus fromi
tlle othelcr, was estimated at.157,000 miles.
392. Tn mtgnit dlce and brigthtness, cmencts exhibit a great
diversity. ey'lc are sometiles so bright as to be distinctly
Visil)lo in thle daytimc even at noon and in thlCe brigliltcst sunshlie, as was A tlhe case with that of 184t-3; and-sulch was the
comet seen at RRome a littlef before:the assassination:of Julilus
Ca1sar. The comet of 1680 covered an arte of: the.heavens of
9?t: andlits length wtas estimated at 123,000,000 milcesi'.' -That
of -1811 had a nlelclus of only 4t28 miles ill diameter, but a tail
132),000,000 miles long. t HIad it been coiled round theo earthl
like a serl)ent, it wouldl have reached round more thaill 50000
times. Othxcr comets are of exceedingly slmall dimensions, the
nucleus beingl estimated at only 2l5 miles; and some whllichll are
destitute of any perceptible nuclels, lappeal to the lhrget
telescopes, even wh en tnearest to us, only as ta small' spl clk cof
fog, or as a tuft of down. I'lhe majority of thlese bo(ies canll be
seent only tby the aid of the telescope.
The same comet, indeed, has often very dificrent aspects, at
its different returlts. ItHIalley's comet in 1305 was (lescribedl by
the ]historians of tlat ag,cC as comieta tor-rewdt m oagit'?aiui5s;
in 1145t its tallt reaclhed from the horizon to:thCe znlitlh, and
inspired such terror, that, by a decree of the'Pope of J.Roml,
1)ub)lic p'rayersl':were ofifcred lup at noonlay in all tlho Catholic
chtlrclmes to deprecate tlhe wrlath of heaven, while in 16(82, its
tail -was only 300 in lletngth, and in 1159 it was visilble onilj to
the telescope, until after it lad passed its p)er'ilhlion. A.ti its
rcccent return ill 1 835, tihe greatest length, of the tail Was abolut
1 ~.:: lThese cltnlges in the nappearances of thlle same comet
(le partly owing to t]he diflerent Ipositions; of the eairth with
* Arago.' j Mimt's Pjizej Essia y on Comets.
it t: llmight be seen mucllh1I longer by indirwcd vision. (Prof, Joslin, Amr. Journ.
Sicience, xxxi., p. 328.)