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

238 T'ril PLANEs.
Fl'rom t his table it appears that the sun consists of matter
bitt little heavier than watert; but that the moon is more thian
three tim-tes as lceavy as water, though less dlense than the
earthl, whicll is five and a half timnes heavier thalln water. ft
also appears tlhat thle nearer planets are more dense than thle
more remote. Merlcury is heavier than mnosttmetallic ores,
while Saturn and IN'el)tlun ale one-fourtlh ligllter thlan water.
The l density, however, does not, in all cases, diminislh outwadtl;
for Vcenus is less dense than the earth, and Satun tn ltan
C I Ali X I I.
PERTUU BA TIONS 01? OFITHE PLANETrS- STABILITY OF THE'I SYSTENU-1 —
MRI'rU l,''Alt: rAS 0. TiE' PLANETS *os........... P AS.
382. T'lxJ, )erturtbahtoms occasioncd ill the motions of flc
planlcts by th}tir action onl each other ar'e very numerous, sitce
every body ill the system exerts all attraction on every otlter,
in conformity withl the law of unlliversal:g'ravitation. AVenus
anid Mtars, alpproachinl' as they 1o at times comparatively near
to the earth, sensil)ly disturb its motion1s; andt Jul)iter and
Saturnl, althouglh very far asunder, still, in conscqueltnce of
their great masses, exsert on eacnl otler, whlen onl ttie samie side
of tlie lceaRvems especially, a decided i;nfluence. M:Xoreover, the
sunll, by his,uneqital action ol the several planets, in consequence of the peculiar figure of each, produlces various irre'ltularities inl theiit motions. As ill tlhe case of the earthll and
mnoon (Art. 2;3), these perturbations are dividled into periodical and secular' perioit/ic/l, when completed ill Cmlll)arat.ivrly
slhort periods, as tlose, f1'r example, wlich undergo1' all their
clhtunges during; one revolution of the planlet; anld securt/,l
whlten completed only in very long periodis, as tose wlichl
afifect th tc fo1rm and inclination of the orbits.
383.'If the only bodies in tlhe sstem were\ a central body
like tlhe stu1nl ad arevolving body like Vtnus, tlhen, when tlho