I N T R O D U C T I O NV
earth about-one o f its diameters- If, there Fore,_we had
so other evidence,* we fatisfied thart • the apU:
parent diurnal ^ motions o f ah the' heavenly 'bodies 'are
.produced by the earth’s rotation.. But vv‘e'h;ave|other
reafons for this fuppoiition. Experiments prove that
all the parts,of the earth have a gravitation towards
*each other. oueKl a5body ;th^r^foi‘e, |tfre ^greater part
“o f whole-furface is a. fluid, mu ft, from the, equal gravi-
tatiQn pf its parts onlyy form itfelf Into a fphere. But
i t appears, from_menfuration, that the earth'is not a pej-
fe6fc fphere but a fp hero id, having its- equatoriallonge^
than its polar diameter. Now if we fuppofe the earth
to revolve,' the parts mpft' diftant from- its axis mufti
from their greater veto city,-.have a greater tendency to -
fly off frpra the axis,ia;nd therefore that diameter which,
is perpendicular to the axis-nkift be increas'd. T h a t
this muftrbe the consequence appears- from this experiment,
that if you take .a. thin iFpn.hoop, and make it
revolve fwiftly abou t one o f itsi diameters, that'diameter
will be diminifhed, and the diameter which is perpendicular
to it will be picre^ed. fjThe figure o f the earth
therefore, wMqh is thaf o f , a fpheroidflattened;, a- little
at the. poles, muft have arilen from its notation.
Another reafon for the earth’ s rotation, is from analogy.
T h e planets are opaque and fpherical bodiesj Ifke ta
our earth; now all .the planets, on which fuflicient ofi-
fervations have been .made to. determine the matter, are
found to revolve about ah axis, and the equatorial dia--
meters o f fome. o f them are vi&bly vgreater than the
polar. When thefe reafons, all upon different prin-?
eiples? are epnfidcr^d» they amount to n proof o f the
earth’ s rotation about its axis, which is as fetisfa&ory
to the mind as the moft direct demo nitration could be.
Tkefe* however, are; Dot ah the proofs th f t might he
offered; the fituations and motions o f the bodies'in. our
fyftejnj neeeffaHjy require this motion o f the earth. It
is no obj eft ion to the earth’s rotation that we do pot
perceive-it.; for-we know b y experience, that when we
are in the cabin o f a fhip on fnaooth' water, i f the flpp
turn roipd we do not perceive its motion, and' all the7
fixed bodies on the ihore appear to turn in a direction
cqntrary to that off the fhip. And in like manner, the'
earth turning about its axis from weft to eaft, all the;
heavenly bodies appear to move from eaft to weft; I t
has alfo been ohjedied..to the earth’s rotation, that in
fuch a cafe, i f a ball' were thrown perpendicularly upwards,.
itjought, to fall we ft ward o f th e place from wh ich
it was projected.,, B u t it is to-be obferved, that when
you; project the ball upwards, it partakes ojF the earth’ s
motion,, and is carried on with it ah tke, ,ticae.it is riling,
fp as to. continue direftly over the place from which it
was proje&ed. This may be exemplified b y letting
fall a ftone from the top o f the maft o f a ftpp4 n motion,
for the ball Jalls as near- to the foot o f th e . maft, as it
. would.do: i f the Ibip were at reft. Or .when y ou are
riding-in a carriage, i f a, ball b e let fall from the top, it.
meets the floor a t the point which is dire^Uy under that
from wh^ipe. it fell.
4. The magnitude of the earth comes next tp be confideredj;
and as the figure o f she earth is very nearly- that?/
'of arperfeét fphere, we may, for qur prefent purpofe,-
confider it ffs ffibh. A n d here we muftpremife, that'if
a fphgre he cut through by a> plane1, the feftion jvill be
a circle if the plane pafs \th rpugh, the "center o f the
fphere, ^he'fe^aonjis called a.gf&at circtejb^i^it do not ■
pafs ^through the center, it is called a fmall cjrcle. AJfo,^
that jfoint o f the heavens which is directly o'ver the liead'^
o f the fpeól’ator, is called his Zenith;, and the ojppofite-
poifit, or that1 dire&ly. under-his feetj^is- ealled. hü-
Nadir*
. X e t P A p E represent the earth, Cit&çeefcet, PVp*
tne^ axis, about } which,- i t turns 5 then the, extremi-
ties P , p 9 arerjealièd Poles,. 5 one, as north pole*
and.the o t h e r , p o l e ; and-all-the; great circles,
as PJpEy paffingthrough the poles, are called*
Meridians, Now all circles arç fuppofe^ to,:be,divided
into 05Q equal parts, called degrees^ -, bvery^egree i&tciy
lip equal-parts, called minutes ; and evçiy mmutç into 60?
equal parts, .palled féconds and degrees, minutes; and
féconds, are .denoted b y -thefe phaqa^era,.^,', thus
37^* ' ,mjean8,c:| 7 degrees,, K
çpnefe. A n d the angleç at the -center, o f the dircJe:Cor«P!
refponding to the arc's, are called angles o f fo many de?
grees,. minutes, and féconds. From G draw the right
hne’4ttfj $0 a fta ta t x ; then tbe ftar *TS;m-.the;zeni.fch.9f
a fpeiffcator at a ; t a h e - ^ ~ , i ° , and draw Cht to the
heavens .at t, then t. is the..zen;ith to^asïpe<âator;at ^ ;t
alfo, tl>e angle. aGh^ or sCt9 is ƒ then becaufe .
the radius •Ch o f the earth bears no fenftble^.proportion .
to the'diftance- Cs o f the fixed ftars, the angle sbt willv;
not fenfibly differ from the.angle sCt, or from l ° ; there^-.
fore to a fpeiftator at b, the ftar j will be; one degree •
from his zenith L e t ,an obferver.-therefore move from .
a i o b:t vtili he finds,; from .0bfervation, .that the ftar s is
i° from'' his zenith, and then he knows;, that die.hah
moved ï° upon the furface o f the earth. , L e t th e di£^
tance ah be meafured, and then you get the length o f an
arc o f i.°, ; and i f you multiply that by -36.0,- the pro»
A- 1 ' ., du&,
I N T R O D K T Ï O N. t«
»3u ft will give you the citctimferè'nce o f the earth. -An 5 W e have already obferved, that the fearth is not a
*ro *&y o f -ady number o f degrees may be taken, and perfeft fphere bv f ? fgheioid, having the polar diameter
then its length bein'g meafuréd, the length ofi.i degree | ‘lhortcr than the equatorial; and the ratio o f thefe dia-
may' be fotiSd-by propörtióni Or, inftead o f fuppqhng meters has been determined?; by different methods. I f
-the ftar to have been in the zenith o f the fpedtator at o;:.’- thé' length p f a degree a t two places be found b y menfn-
^ve might have taken a ftar at w; and the difference be- . ration^ tliat '-</ai«>H is fufficient to find the ratio; but the
tween th;é èénith di'ffan^, Vof 'tBe 'itar 4 at the places a. ratio thus" detei mined’, by taking different meafuremerits,
and would have been th'e. fame-as that o f the ilar r ; I differ conlideraijly'. hlrti-Vinc e, in his Complete Syjlem of
■ fo that when the obferver had moved over an arc ab o f Ajlróiufnp, ''^e^^l P?ge !§Ê?has determined the ratio
:the kthilh’diftahée'hf'bhefftar- Vi great ; and it will be found
1°. In this manner tlie 'length o f a 'degre e'^^ïgre at. s that they differ conliderably ; but the mean o f the whole
circle upon the ea:rt%,V' fn'tfacé;hds :been 'i'ff^ttifthmEd;V’ giveiltlie.tatid ovf for the •proportion o f the
and thence;: ii¥s citclUnfefence. a P ossidonius, . who polar to,;the equatorial'diameter o f the earth.' Slit I .
lived in the time' p f P ompey the ^r«/, attempted ithus N ewton, from the principles o f griaA'itatioo, m^kes the
to meafurerthe circumference of the earth ^ ratio. 22.9 : J 1 fomejauthors have deduced a.mesn
the ftar called ..(Smc^Kr was in the horizon at Rhodes,, ratio from mensuration, ’:;w;hich agrees 'very nearly with
-ihfid.tftat at V#lexaijdt-ia its altitude ón the meridian was this. T he 'lén^fhj&^Hcndulum vibrating feconds, in-
y-p ; and the diilance befiveéh the two places ' ( they- : creafes as you carry it Éowirds the;poles; and.this ought
being neatly in thé fame meridian) was 5000;j?arf»a'; to take placi-ah confequence o f the fphefoidieal'figure
whence he concluded the circumferenpe o f ’the earth to of 'the earth, as before determined, 'and affords another
'be 2i|:e>,’óóo jW in . .•Euf’ as. the exaft. ^junVof the! proof ó f that'figure. A n d if the length o f a pendulum
' Jladia is .not n'ow known, w e ’ cannot fay h o x 'a c eu tke vibrating.feconds in two latitudes could be accurately
; this cohclufion 'is.' :'Óii'f’ 'cöiihtr^nian.MR. N orwood afeertained, we might find the ratio o f the diameters o f
in dteterfifne^the va:lde die'earth, the denlity of the earth being fuppofed unic
f a degree to ' a confiderable accuracy. 4 H é 'took the •' form. But the ratios thus deducedifrOm different obfer-
height ó f tlie pole ftar at Zé»don'an d'a t'T o il ,• and b y various, differ confiderably ; owing, probably, to the
tm’afuriiig ehei diilance, he determined the length of-a irrcgulai ity o f the denfity o f the interior parts o f the
degree to be iig i ’ miles and I4 pdiesl A f te r 'truit tifiie, earth. .jVIiXI liaIr *f.rj’!n-ohferves, that >the> variations o f
h?he.‘T''r/eneh ■ ^4ad'e,my; meHured''aiidegreè. (yq’ffini'hiek- Jthé lengths o f pcndulums.rtaake the ratio o f the diame-
furéd one in jVance ènd^fteriVarfl'.è’AirAii1, Maupet- . tefs ,n«arer, thariof equality than 229 ; 230, itiótcatirig
/Vii. UikI fevferal other, eminent TnRtiïèrrtaHfciaiK., mea- a greater denfity towards the center. It'lias' been alfo
fured a degree ié 1 upland. T he fahïè' 'th&vmrements 'plrjp<)fed -to find the ratio o f the diameters o f the earth,
have been alfo frequently -repeated in' various parts o f from folar eclipfes, as the computation or the parallax
th e earth, anil .thé réfulfc o f tlie whple'Ss this, that the o f the moon',' and confcquently the rimes o f the vbegm-
dstlgöï'df h'dtgre&, as yo^fretebn t'r th^ ^ u a fo i-törtlïe ning and end »£ iïiïèh .eclipfes, Will "vary,.accordifig as
■ poles, increafes in léngth.' dNow the longer i. M i l , the rario o f the diameters o f 'the earth vary. M; de-la
the greatef-muft be the circle o f which it is a pa rt; aiid H ano’e fro'm éhencé- makes the difference o f the. drame-
ithe greater the~circie is, the left is its curVature. It ters to be Ti-Q- o f the whole. From a confideration. o f
appears therefore from aftffal mehfdrauon, that the all the circumftances, it is probable that the difference
' earth is flatter, or o f lefs curvature, at the poles, tlia'n udf -the gpdlar and equatorial diameters is -lefs than that
a t the equator, agreeable io 'what we. before flowed »Which is determined b y 'SiR I- N ewton. I f we take
-feuft(n|kffarily be the'.confequehce.óf fhé -earth’ s rtftf- the ratio o f .the-diameters as determined by him, the
’blow ■ ' -TK e lk tfth o f a .degtfet%f/larièud‘è 5j.*;b‘ lS ‘S q h e^iiatoml diameter will be found.ta eitceed -the polar,
lEngliih miles, and .'this we may conhder as a 'mean, . by about 34: miles,. » ^ ' ' '®
lengjth-; Hence, -6^2 x jfio i= i^ ^ m i l e s , - the cir- '> 6. I t appears by calculation, that when the eye Of'a
%htoference o f she earth ; and ak the*e®tÖmferé*®' b f fp®ftator is 6 feet above the furface e f "tti’etfea, he can
tevery cïfclH'is to its radius as1 R{28318 to 1 , we fee 3 m i fe ; -and at any other altitude o f the eye; the
y&Ve, -6,21!3.1,8'; 1 : : '24912: miles, the radius diilance at which you can fee, vahes-as the fquare root of
o f the earth. D r. L ong veftimated the proportion o f the-altitude; i f therefore o.be the altitude o f the eye in
■ land to water upon’the furface o f the earth, fo far as fet,\and;rf the diftancejn miles, which you can-fee: at
■ difidiftties had'then beeh made, in the folio wing-man net. that altitude, then : *d ~ ■
H e took the paper off a terreftrial globe, and theh cut out . ' > ■ ** ' v / 6 X
■ the land from the fea, and weighédthe^two ; b y this 1 Hepce, we havq^ tlys
means he found the proportion o f the land to thé fta rule . Multiply thefqüare root of the height of'the eye 'ir,
as 12 4 : 349. Thj* conclufion would be more accuilate, 'feet,hy 1,2247, andtheprodiiB is 'the iËJlance to nohicfj.you
i f the‘land*'were cut from the fea. before th e paper was can 'fee in miles. For example ; i f the height o f the
P«t upon the glbbê. ’ A f te r all the modern difeoveries, eye be z / feet, then the fquare ( < 5 , and
this method would probably give the proportion of land ; f HW multiply 1,2247 by 5, the prod'uft • is '6,1.235'
-to water, to a confiderable degree o f accuracy. , < miles, tlie diilance to which the eyé can fee. ■
■ bi . On