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

OItAPTEIR XIII.
COMEtS` flt'f''' tgr1sOXt0 8B.fO.OEtf
390. A Cot;r,: wbi cifll perfectly form'dtl, onlsists of three
parts.. thte iN ucleus, te Enllvelo)e, anld tile.Tail. The N i-ucleuts,
or blody of the coimet, is genertally distinguilshed by its-forlilng
a bright point in the centei of thle lead, conveyintg tlhe idea of
a solid, or at least of a very d(ense poltioln of matter. Tholugh
it is usually exceedingly small when compared with the othler
parts of the comet, yet it solcti!mCes subtenlds an anagle capable
of beinlg inenasured by the telesco)pe. Thle.Ktvetope (sometimes
callec the coma) is a dense nelbulous covering, wtllic fi'equentlv renders the edge of tle nucleus so illndistinct, tllhat it is
extremell y lifflicult to "ascurtaill its dia.etelt with any degrisee of
precision. AMany comets have to 111loeus, but present only a
nebulous iass extremely attenuated on t.he confines, biut gladlIally increasisng in (lensity toward the center. Indlee(l, there
i of os a regular gradation of comets from such s re composed
merely of a gaseous or vapory mlcediullm, to those whicll have a
wcll-dcilfincd nucleus. In some instances on recordi, astrono
mer1s have detected withl their telescopes sall stars tllrougltt
thle densest part of a comet. The.7.il is regarded as an expanlsion or prolongation of the coma,; and presentingt, as it
sometimesl does, a train of appallingf lagnitulde) and of a pale,
portentous light., it confers oi this class of bodies their pefculiar celebrity.
391. Tihe number of comets belonging to the solar system,
is probabl)ly very great. [lany, no (loubt, escapre observation
by being llabove thle horizon in the daytime. Seneca mnetiols,
thiat duri-ng a total eclipse of tlhe sun, wthich hap)pened 0G years
beforc, tihe Clristian era, a large and csplendid comet suddcinly
made its appeairance, bein, very near thle sl. Th'le celements
of at least 1.80 comets hlave been computed, and arrangedl in a
Kdt4a, cotmar, from the bcardd appearanle of comets.