G E O G R 
■whatever part of the earth it is, tlie meridians and pa¬ 
rallels of latitude may be repi efented by eqiiidiflant pa¬ 
rallel lines. 
A line which cuts all the meridians at the fame angle, 
is called a rhumb line; as long therefore as a tliip tails 
upon the fame rhumb, it fails upon the fame point of 
the compafs. When tlie projedlion of the meridians is 
by circles, then the rhumb line is a curve; but when 
the meridians are rcprefented by llraight and parallel 
lines, the rhumb becomes a ftraight line, it being' tiie 
property of a ftraight line to cut parallel ftraight lines 
in the fame angle. Hence the great" ufe of Mercator’s 
chart, whlcli is contlrufted upon this principle. Upon 
the earth’s furface, the degrees of latitude are all equal, 
but the degrees of longitude decreafe as you approach- 
the poles. Now in this projedlion, the meridians being 
equidiftant draight lines, the degrees of longitude muft 
be every where equal; in order therefore to preferve 
the proper proportion between the degrees of longitude 
and latitude, the degrees of latitude are increafed in a 
proper proportion ; the degrees of latitude therefore in- 
creafe as you go from the equator to the pole. In failing 
from one place to another, the fliorteft way is to fail upon 
a great circle, but that is a thing which is imprafticable, 
there being nothing to direft you in fuch a courfe. Na¬ 
vigators, therefore, when they have to go from one 
place A, to another B, find upon what rhumb they mufi; 
fail, that is, upon what point of the compafs they mufi: 
go, fo as to come to B, and by their fteering compafs 
they can tell when they fall on the fame point. On Mer¬ 
cator’s projection, if you draw a ftraight line from A to 
B, it gives you the rhumb required ; for in thefe maps, 
there is a point afliimed, and from it there are drawn 
32 (traight lines to the 32 points of the compafs ; when 
therefore you draw the ftraight line from A to B, you 
muft obferve to which of the 32 lines it is parallel, or to 
which it is neareft fo, and you thus get the rhumb, or 
the point of the compafs you muft continue to fail upon, 
in order to go from A to B. For inftance, if you find 
the line A B is parallel to the fouth-weft line of the com. 
pafs, then if you continue to fail on the fouth-weft point, 
you muft come to B. 
In all maps, the upper part is, or fliould be, northern, 
the lower part fouthern, the right hand fide is eaftern, 
and the left hand fide is weftern. On the right and left 
fides, the degrees of latitude are marked; and on the top 
and bottom, the degrees of longitude are marked. When 
the maps are very large, the degrees may be fubdivided 
into halves, quarters, &c. 
When the meridians and parallels of latitude are 
Jiraight and parallel lines, the latitude of a place is found 
by ftretching a thread over the place, fo that it may cut 
the fame degree of latitude on the right and left fide of 
the map, and that degree is the latitude of the place. 
And to find the longitude, ftretch a thread over the place, 
fo that it may cut the fame degree of longitude on the 
top and bottom of the map, and that degree is the lon¬ 
gitude of the place. For inftance, if we take the chart 
of the Eaft-India illands, and ftretch a firing over Siam, 
we fliall find that it will cut each fide at 14° N, lat. and 
the top and bottom at to° lo' E. Ion. Thefe therefore 
are the latitude and longitude of that place.' On the 
contrary, if tiie latitude and longitude of a place be 
given to find the place, ftretch one thread over the given 
degree of latitude on each fide, and another thread over 
the given degree of longitude at the top and bottom ; 
and at the interfettion of the threads is the place re¬ 
quired. By this means-you may put down in a map, 
any place whofe latitude and longitude are known. 
Now let the meridians and parallels of latitude be 
curve lines. Then to find the latitude of a place, a parallel 
of latitude muft be drawn through it, by the fame rules 
as the other parallels are drawn, and it cuts the fides at 
the degree of latitude of the place. And to find the lon¬ 
gitude of the place, draw a circle of longitude througit 
it, by the fame rules as the other circles are drawn, and 
Von. VIII. No. 509. 
A P H Y. 36i 
it cuts the top and bottom at the degree of longitude 
of the place. But as it is troublefome to draw thefe 
circles, the following method may generally be fufficient- 
ly accurate. To find the latitude, find by a pair of com- 
pafles and afeale of equal parts, hovr far the place is from 
the two parallels between which it lies, and divide the. 
diftance of the parallels in that proportion, and you get 
very nearly the latitude. Suppofe, for inftance, the dif¬ 
tance between the parallels to be 5”, and that one is a 
parallel of 45'’, and the other of 50" ; and fuppofe the 
place to be within 3 parts of the parallel of 45°, and 7 
parts of the parallel of 50° ; then 5° muft be divided in¬ 
to 10 parts, and 3 of thofe parts muft be added to 45°, 
and it gives the latitude. This is done by proportion, 
3 X 1 5 ^ 
thus, 3-P7, or 10 : 3 50 :-—-— ig°; therefore 
the latitude is 46nearly. In the very fame manner 
you may find the longitude nearly. On the contrary, it 
the latitude and longitude of a place be given, to find 
the place, draw a circle of latitude through the given 
latitude on each fide, and a circle of longitude through 
the given longitude at the top and bottom, and their 
interfedfion denotes the place. 
To PROJECT A Map of the World on a plane 
SURFACE, ACCORDING TO THE IMPROVED GLO¬ 
BULAR PROJECTION. 
To draw the circles of latitude. —Defcribe the circle N 
S E, Plate II. fig. 7, of any convenient magnitude, re- 
prefenting one hemifphere, or half of the earih’s furface ; 
draw the diameters N S, and E Q, perpendicular to each 
other. The line E will reprefent the equator; NS 
the axis, or firft meridian. Divide the quadrant QJS, 
into nine parts, as lO, 20, 30, &c. or into fmaller parts 
if the circle be large enough to admit of it. From E to 
each of thefe divifions draw right lines, as E,y', 20; E, 
f -, 30; E, d, 60; &c. Divide into two equal parts the 
ines f, 20; g, 30; d, 60; &c. and from c, th.e point of 
divifion, let fall the perpendiculars c F, c G, and c D, 
produced till they cut the polar diameter N S, extended 
indefinitely inj)'; then the points F, G, D, will be the 
centres from which the circles of latitude z,f, 20 ; z, 
30; z, d, 60, are to be deferibed, or ftruckwith the com- 
palTes, which will be the true reprefentations of th.e pa¬ 
rallels of 20, 30, and 60, degrees of fouth latitude. In the 
fame manner draw the parallels for every tenth or ffth de¬ 
gree of latitude.—To obtain thofe in the northern liemi- 
fphere, fet off on the line N S, produced in the oppofite 
direftion, the diftances S D, S G, S F, See. which will give 
centres on which the circles of latitude are to be deferibed 
for every tenth or fifth degree in that hemifphere. 
To draw the circles of longitude. —Divide the quadrant 
EN, into equal parts, 10, 20, 30, &c. Divide the qua¬ 
drant S Q, into two equal parts at s, of forty-five degrees, 
each, and let fall a perpendicular s s, from the point s; 
fet off on the line N S produced S x, equal to s s: then 
lines drawn from x to to, 20, 30, &c. in the quadrant 
EN, will divide the radius in the points 10, 20, 30, See. 
through which the circles of longitude are to be drawn. 
—Here the geometrician will recoiled! that a circle may 
be drawn through any three points, not fituated in a 
right line : and the method of finding the centre is to 
join the three points, then bifedl or divide in two equal 
parts the two lines fo drawn, and from the points of bi- 
fedtion, let fall perpendiculars, which being produced, 
will cut each other, and the point of interledtion is the 
centre.—See this demonftrated at fig. 5, in the Geogra¬ 
phy Plate IV. 
The next thing required, therefore, is to find the 
centres from which thefe circles may be drawn.—To find 
the centre of the circle S, 30, N; join the points S, 30, 
and N, 30; divide into two equal parts, the two line* 
in 0, and let fall the perpendiculars 0, 30 ; 0, 30 ; and the 
point where they meet is the centre of a circle of 30 
degrees of longitude. The other centres may be found 
exadtly in the fame manner.—Or, the centres may be 
4 2 fouaii 
