BHH
On the Latitude and Longitude of Places upon
tbefEartb's Surface*
7* L e t P j f p ^ reprefent the, earth*- PGp its axisjiiP
. the north pole, p the fbuth pole ; and let a
circle-paffing through the center C i perpendicular to the
axis Pp, then that circle is called the equator. . 'T h i s
circle divides the eatthinto two equal parts, A P ^ called
the northern, and ^^^jcalled the foùthern hemisphere.
L e t K y Gy ly be the fituations o f three plàée&^tipoB ^the
furface*.and-through them draw the great circles PKp,
PGp+PJp, called meridians, interfering the eqtator -m
n% a9 nty rdfpeftively*iL Now . as every circle-is fuppofed
to . hé divided into 360 degrees, from the pole to the
equator muft be 90 degrees- T h e latitude * o f a- place,
- is an arc of its meridian intercepted befween the y place and
the equator, meajured in degrees. Hence, the latitude,of
K is measured b y the degrees o f the arc nK.; . and . the
latitudes o f G and / are meaftired b y the degrees o f the
arcs aG * m l, refpe&ivdyy and jrhefe are called north
latitudes, the places lyin g in the northern hemilphere ;
and the latitude o f W is jneafured b y the degrées'of the
arc a W , and is called foutb latitude, the place lying in
the fouthern hemifphere. L e t the fmalL circle cGvde be
parallel to the equator, then this circle is called a parallel
o f latitude, becaufe every point o f it has ; the fameJa-
titude, all the arcs mo9 aG, intercepted, between it .and
the equator, being equal, on account o f the circles being
parallel. T he longitude o f a place is meafured upoR the
equator,-and is the arc intercepted between th e point
from which you begin tô reckon; and the point where
the meridian o f the place cots the. eqnatqr,j-e(|tmated
degrees. Hence, all plates in the fame meridian' hafe
the fame longitude ; the longitude o f G is the famé as
the longitude o f W. Geographers o f different countries
begin to reckon from different points, each qegiii-
ning.fyoiBi that point where the meridian o f 'its capital
city cuts the^equator; and iLth^jjcify-have,
obfervatory pi^or. very near to it, that ïner^dian is-,taken
which paffes ^ rou ghs the obferyatory. vThis is called
thejffr/f meridian. ^(em^y (therefore. 4efihe the* longitude
o f a place tq-be an are o f the equator intereepted he -
tween thefrdmertdiqn an^the mer/dian pqjfmg thpoughjhe
place. Im JSngland therefore we begin from Lhac meri-
■ dimi which paffes through tjïe qbforvatpTy at; Greenwich ;
in:France, they begin from that meridjan'j which paffes
>thrpngh the obfervatory at Paris. - L e t j ‘therefore Q
: réprefent the,rqyalj-obferyatppy &\Greenavich% ai}d q is
the point o fth e equator .'from w h i c h . w e t o reckon
the longitude r\ Hence, the degrees o f the arc an\ is the
h^tgiihde of the place / ; apd th^ lp a^ tu d e (lpf.^tbl^ -
. place K is meafured b y ‘thé degrees o f the arc <w7.LjN q\v
-the direfiipn pm from a as eaft, - and the,dii;e<ji6n~u/iiis
%$.*: it is therefo^e, u£uaV tp palivi4^ i^ 'fep g i^ d e , land
tanjeuefi Jongitpde, epch till-yop co^j^tö the point ©pip©---:
Aftte/to a, o r tilLthe ,lpngitude,.»^ch ^w%y^ ‘
.degrees. " B u t fometimes the longitude is reckoned all
jj*be .way-round in the (fame- direction; that is, the point
m, wherever.it may, be, is a*
I f the latitude and longitude o f .
the,,placejitfelf may be~ lo^md^' for if the, ïjon,gitu.de
known, let off the arc am it be eaft lop'gi-
_-tade, and draw the, ^ mPS: th e i afflhLlptitude -
be north, fet off ml equal to> it, and / is-the place re-
jj quired; =buvt i f the latitude be fojfth, ie t’ p’ff-wz/^.^qual
to itr, and V is thej place.- I f thè-4ongitude bp weft,, fet
off an ei^urf-to.it, a n ' d t p ^ t h e -la--
will be the place. > Thus, alLthe places uponthe,furface
t pLthe earth, whofe lafitudes-^aiidvlongitudes are.known,
may.. Be laid down accurafielyj'upon a tglobe and the
' boundaries o f the different countries may b e ,traced out,
and each-exhibited, in its ^r^er^fit.uati.on and' figure,
i B y means o f a globe therefore you-.may g e t a perfect
idea o f the relative magnitudes, figures, and fituations
o f ail the .countries o f the earth", a n ^ lf ;|the"fituations
q lp ll the principal places in ‘mjng||; ^ u t '
a plane furface, cannot correctly, reprcfentvtheir proportions,
boundaries, ,^nd politionsjof^^^pl^c^s.^ v T he dc-
termination of- the latitude and longdiSe Ja ther,efd^e
effentiaLtp geography,^and:confequently to navigation;
. the methods, by which thefe. are. found; we -fliali pftec-
wards fully explaiirf.,
9. T he arc Gv contain^-the- fame number$i,;deg!^8
' .as the arc am.; the degrees, o f longitude therefore { between
any - two places, when . meafurpd ^mpon. ^a fmall
. circle parallel, tp the equator, diminifli as. that circle .approaches
the .pple. T he .arc am contains the fame
bgr o f degrees as the Jtnigle. aPm.j hence,, the wangle
formed by the meridians paffing through any two places,
is the, meafure o f the difference ro f . the , longiku4§a,ipf
thdfeplacjes;.
^ig^f'I’he^ following Table contains the lehgth, o>f a
degree ó f } longitude in £nglifh miles fort every degree o f
latitude..
Lat.
Deg. o f Long. ILa t. ©eg. o f -Long. ILa ti liBeg. o f L a t J :Dég. ’o f Long.!!L a t. IlDeg. o f Long. I
69 jk'OGlO ■ -65,8134 1N * » 55.98 4 ^® ... ■■'.4o;6751 ' J ^1.3842
I 69,1896 s t 'b a qN M ) i 37 5ï.? ö ’59' 55 1 ' ' 39i69:‘ 7 ■ Ii 7 3 1 29,2320
1l
’■ 2 i ' 20 ,i 1 38' . 3 8 ,6 9 5 9 " 74 i ■ P 9 ió743.
3 - ■ ■.■ ■ :69,foy2 ■>mjmt 64,603-7 f i f 1 53-,57b»" - f f im ’ 37.6S91 I 7J 1 - i j , 9 l °3
.. • '4 .^'■ rwiogi* <. -i î ?sf.' ■ .'6'4,,i6;99j,,.iti'' ■ 53,0109 m ór..-i6,7409... '
! ■Kyâ-. ■ 5 l 1158.9363V*., :. .63,6986 : -4> 52>225.9 ' f * » ; ï 77
■ '6^ f024,; : 4Z - ■ 5 ‘ f 53 ■ ' Ifêô.î;. ■■■ 34,6000: Il 78 f -14.3874
■ 7 ■' 1 1 f . iB i 62 - 1&7» • 4 3 ' 5 c ,6094 . f- ■ 33.548;9 • 79 1 I3.2041
l l ® '- i l i s S ö r ■ 62,1963 ' 1M l -49.7783 ': - '62:' ' 1 l33'.4s 7 3 :J ;; •80 S 12,OI66.
" 9^. 08,3.38 t.. 1 61,6579 g t W >. > 48,9313 .. 63. fi 3i'i4 t6 i ; -■ S i ' .10 ,8 2 5 0 '
IO 61,1489 ’ ■ ! r'¥4 ' vi' 64 ’ 3° ;335.?: ' I 82A, . ',916306::,/
11 ■ é^;z8.8ï4 . 29 ■ 1 47 47r f94 I s f e l 2952453 83 ' 8,43.34
‘ l i ; j ■ ' ' 1 y ï .ipjô' 59.9293 * |<4 8 - - 46.303S ■■661.; t- 28,1464 84.; V’z 335
' r67;»4264. -. ! ïî* ï "49 45.399'4ÿ*-': ■ f Si 2-7-,0385 85, 6,0325 : ;
5- 0.67,1448 M ' 58.0851 1 ' 44,4811 m ë j ' .86,0■ y ■ 4,8 2 7 4 .. V.
8 i | p ‘ 66,8424. & a i ? t - jt8,P36o ft i fc ? U 43.5489 « ! ■ ' ■-■'■2'4 7 9 9 2 '-' ',<I-871 13.6219 •
K f fT 57,3696 W m ; • 42,6037 ; 1 7° 1’ 88, ^ È n S È È
I I S s l 1 6 6 ,1 7 6 0 - '; V :&-i ~5.6,6,?sz - Il 53 1 4 ‘ .6453 ' 7 1 l 22,5294::.'; S U . . - ;
{, On. ifrevAtnwJpIpiYe] 'pf the. Earth .
1 1 . T h e earth is furrflunded'.with a thin, invifible,
elaflic fluid, called a«-, the \vh®le body o f which forms :
what- is called' the atmofpliere. I t being an elaftic fluid,
is capable' o f compreffion ; on“which account, the lower
parts o f thfe atmdfphei-e are denfer than the.upperparts,
and the derifity .gradually diminiflier, the higher you go,
from the continual diminution o f compreffion ; for the
air being , found to have. weight, as' you afeend, the
weight o f the incumbent air- will be diminilhed. The
denfity o f the air is not always the fame, it being fob,
je f t to be expanded by 'beat and contracted b y 'c o ld .In
tliShWater.^ut^pbtwithfahding the air is fd;&ttremee
ly rare, it is caphhlel&f ^odubingVery, ^confiderable' ef-
fefts upon the rays o f light as they pals through it,
both by reflexion and refraaiori."1 B y TC^ejaion, the
t rayseoiming from the •futfrKl'liilg on 'th^pa.i;ticlest&f aifj
and'Uppii the vapoufs-nand exhalations rfcontaihed ^in
tiie at mo'fphfefei are thrown1 in. all
the- whok*'hcavens! Kectjfoe illun-iriatcd; by which our
eyes are -ai&aed (a llrongly,'as1 :to render- the'fainter
light o f fWfta rs infenfible. WhertAs, 'ir' there- stert'ub
tliofe fays;-which
jebme tKe'lieavens
would appear dark, and the liars would all be -vifible as
at'night. • From"the fame caufe ireTechivc 4'''cbi>#ftef‘
■ ^ le^ h a 'h tity b fl^ t’feOTbiridtiifi^befofe'lhh fop r-ifes,
and’affer lie lets ; 'this'is called'
not fof:ti'iisi'\\fe*|fliouldib'e mydlVed'ih’ibfaldairkriefs; the
iif^aht ateehtKdfiiMisifet ; and there would be a ‘fii'dd'en!
’ tAnfitidn From darknefs to light; at the riling o f the
'fan, which would be extremely prejudicial to the eyes.'
J-rpni Elrelcim'e ai'vvhich' twilight begins and ends,' the
beginnings and eiid are found to be when;'the fun is
about Ji'S0 below the hbfiz6n..:"o Itia'fts however fill the
fun is further below the horizon in the evening, than
he is in the mornirig/ivhen it begins; It alfo lafts longer
in fummer than in winter, in the former cafe, the
heat o f theTday.iias'raifed the vapours arid exhalations ;■
anff-^tr-thfe'' Bttrri they will-be mbte elevated from the
®ap’- o f : the-foalbn' j arid:-therelore the twilight ought
to h e'Mn^rhn 'fbe ^vening than m-foe morning; and'
longer in winter than, in fummer. •
12. Another property o f the atmofphere'isthat o f
fefra8;ihg‘ fh'e" 'rays o f : light; by which means the heavenly
Bridies' appear out- bBtlWir true places.1'— I t is a
prineTjde'tif optics; that when a ray o f fight paUes oirt
df'4-'t^nfor'lhto'a'forertbeibnihl' !t; ^ bent towards th e
perpen(licular-to.-the furfote' o f the medium- at the point?
where it enters'.' A ray o f light' therefore comiiig from-
arij ef-the heavenly bodies, when it' ttfters.the top o f
■ the ntmdfpliere-'wult be -Kcnt from its reftdiaear- courfe,
to wards' a.' radius1 ilra wii ' .fp® i :earth?s-center, beeaufo
the radius is ptrpeudiai!:ir to the forfece of-the atmof-
ph'ereS'; fond'as, in approaching the earth’s: furfate, tho
denfity oFthe atfaofphere continually jacreafes, the rays
Of light, as they defeerid; are cbnftantly entering a den-
.fer medium, and- therefore the fccurfo o f the ray will
cotitirmally deviate from a right Uue towards.a radius
drawn to the ; earth’3 'eefifcri ahd'';:'deforibe a ' curve ;
henee. at' the furface' o f the earth the rays o f light enter
the eye Of thfe'fpeftatot- in a different direction from1
what ftieyi ijirouM- have a 5wiedi;if there Had been tio at—
mpfpK'efe therefore ; the'/ place o f the b,oByr
frbbtrWfo*(m' theiiglrt 'ddrd^s muff be%ilEsrent from the-
rivie-place j and;as-‘ life milKje -of tW r iy i has been Oslo-'
fm^aily;approacliiifg .to a radius drawn to the center of'
.the earth, its direftion, When. it eotnes :to -the furface o F
• tlie earth; tirull \yc jnclined^fr-pm.its oirginal direction,
towards the zenith ; therefore the
body is higher than its true place. The ancients were-
-not unacquainted with this effe£l::: Ptolemy mentions rr
- diSeTence