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

11ENSITYS OF t. l ItHE t' 6 9
the equator, combined with the incertia of the earth in its rotation onl its axis.
W\c thfls ihave the shape of the earth established upoll the
Inost satisfactory evidtce, and ar'e furni shcd with a, starting'
point firom which to determine various measurements among
the hleavenly bodies.
14t1'. The detcnszty of thle carthl compared with w ater, thlat
is, its specifihc gravity, is 5"a.:. 11he density vwas first estiated
by Dr.'II'ttonl, fr oi observations made by Dr. IAlaskely no,.tstr'onom1er 1oyl, onl Sctlhalliln, a mountain of Scotland, in
tlhe year 1774. Tthus, let A:1 ltig. 2p.
( ig'. 2,5) represent the mloun- ll
tatil, ), 1, two stlations ont op-.~.fi..
-positc sides of tle l mountain, ll
antd I at star; and l et I a111 and
1[( be the zenith distanes as
d(lterllnt ed by the diflferences
of lhatitudes of the two stationls.
Butt the apparent zenith dishtmlcos fls (deten1i11ed 1by the
plunb a-lie are IE' and IG'.
T'lie dleviation toward1 thle ilnountain on eachll side ext ceded 7' ". tt 1
Tlei attractiorn of the mountainl
being observed on 1)0th sides
of it, and its mlass being computed frlom the number of seetions taken in all directions, tlhse data, whent compared witht
tlhe known attraction anld magnitlude of the earth, led to a
knowledge' of its mean density. According to ),r. i[futton,
titis is to thalct of water as 9 to 2,; but later and more elaccurato
estimates htave made the specific gravity of the earth as stated
above. lnt this density is near ly double the average delnsity
of the materials tlhnt colmpose the exterior crust of tie earth,
slhowing a great icrectase of density toward the center.
The density of the earth is an iltnportant eleent, as \we shall
find thZat it helps u11 to a klnowledge of the density of each of
the ot1'her ne101lmbers of the solar system.........................,',. 2 1...........is.............................
o Baily, Ast. Tables, p. 21. t Robis,' Phys. Ast