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

25G (X)MiKT.S>
It appears, perhlaps, incredilble ta so tin a substance shou
lb)e visible by reflecte(liliht, and some astronomers have el
that tlhe matter of comcts is selfuminous but it requires but
very little light to rend(ler anl object visible in the nig,
light vapor may le visible w illate throuhout an
immuen'se stratuill, whlich could not be sen if sprea over t
face of: tie sky lilke a thin cloud. The (ghest Clouds that
tloat in our atmosphere, must be looked upon a des ad
massive l)bodics, compl)ared witl the flmy and all but spiritual
texture of a comet.''Tlie small quantity of mtte i Comets
is further proved by the fact that ytve oeles sed
very e(o, to so0Ine of t11e p1an ets h t hl tii motiols i;n. tany jpreiable degree..,us the Comet of 0 i
its way to tile 1sun) got ntangld ong the satellites of Jupiter, and remained netar tthem four onth, yet it did ot p
ceptibly cllange. tir lot ions. aO omet also came
very near t}heearth;'so ne that, ad its mss ben equal to
tlhat of tthe earth, it would have caused the ar to revolve i
an orblit so much larger than at pesent, to ve crease
the length of the, year.21i. 4-mnvJ Yet it produed no sensible
effect onl thle, length of the y'earl, "and therefore its miass, as is
shown by Lalplacc, couldl not have exceede~d -( of that of the
ear~th, and might haive been less than this to any extent., it
may indheed be. asked, what. proof we have that comets' have,any) matter. The answer is) Iirst, thley. reflect light; second,
though not suiflicient to disturb so he'avy bodies a(-s planets or
satellites, ymet they are themiselves exceedingly distilrbed by) the
action of the planets, and in ex act conformity with thre laws-1 of
univer~sal, gravit~ation.: -A delicate compass mayt) be grveatly,
agitated by thle vicinlity of a. mla's"s of Aii'o11, While the( iron'is
niot sensibly,affected by the attraction of thle needle.
396. B3y approaching very near to: a large planet, at comet
may-have its, orbit: entirelly changed. Thsfact is strikingly
excmplhified in thle history of the comet of _1770. At its appearan11ce in 1770, its orbit wvas found t~o be, an ellipse, requi ring for
at complete revolution: only3 5. ya;and thre wonder wNa,
thuat it had not been seen1 before, since it was a very large and
0 SirJ.- HesllmceI. jai'place.