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

206r:, Til-. P LAN TS.
i.ncollhoency of their parts, if th.ey consist of solid matter.- A
chalngeal)lcnss: of formt and conditiolln:is inlicated, also, \whltc
the ring presents to us its edge vicw.:t solmtimlC appears
as a Idelicate, uniform line; at others, as a, line of:unequal
thick(less, whose thllinner parts occasionally become, entirecly
invisible; and. the in equat l iies elitange thlir aspects, fi'om tinle
to timle, ina mannetr not to. be accountedl for by the revolution
whlichl tlle. systeml is known to. hnave.
A most. rcmarklablc fac(t relating to thlec rings, is their exceeding thlinness. I'licy have generally been relgarded as about 100
miles tick. IBut Bond's observations lead to the conclusio n
that their thlickless is less thlan 40 miles.:f a nodel of the
rings, onelo fot in diameter, were cut out of conmmlon writing-.
paper, tle thickness Awould beG too great to represent tlhel
properly.
343t. Saturn' s ribg, ifn its r'evoution atround Mte sun, alzways;'e5k7ma'Jis p?',adilt to itsel-.
If e. hold, opposite to the C3r, a circular ring or disk, like
a piece of coin, it will appearl as a complete circle when it is
at riglht anigles to the axis of vision; but when oblique to thllat
axis, it w\ill 1be projected into an ellipse Inoreanld more narrow,
as its obliquityI is inlcre8asled, until, when its p)lanc coincides
vithl thle axis of vision, it is }projected into a straight line. Let
us place on tle tadble a lamlp or a ball, to represent the sun,
and, ) olding) the rilg at a ecrtain distanne, inclinled. a little
tioward the ccntlra body, le t us carry it rouli(l, alnays keelilng
it parallel to itself. ~ )1Durilng its revolutioll it will twice p)reset
its edge to the lamp or ball fat opposito points, andl twice, at
90(~ listance fi'om those points, it will present its broadest face
towardl tIm central b1ody. At intermediate points it will exhtibit anl ellipse more or less open, accordini as it is nearer one
or thell otler of tle preceding positions. It will l)e seen,: also,
that in olne-]half of the revolution thce lamp shilnes o(n o1ne side
of the ring, and iin the otlher lhalf of the re-volution on tlhe otller
silde. Suclt would be tihe. successive appearances of Satturn's
ringIt to a spectator on tlme sun; and since the earth is, in 1'slpe:t to so distant a body as Saturn, very near te silnl, thoso
appearances are presented to us nearly in the salme nitnner as
though we vrviewed them f'rom the sun. Accordingly, we some