Mathemati- 
cal Geogra- 
phy: 
PLATE 
CCOLXVIL 
Fig. 6, 
Natural di- 
visions of 
the globe. 
Continents. 
Fslands. 
Peninsula. 
Isthmus, 
Cape. 
Oceans. 
Seas. 
Gulf, 
Bay. 
Strait. 
Method of 
represent- 
ing these 
subdivi- 
sions, 
170 
struction of any kind of maps, yet when it is required 
to represent a small portion of the earth’s surface, and 
to exhib the different nearly in their true pro- 
portions, the following is perhaps the most convenient, 
as well as the most accurate of any. ; 
Suppose it is required to construct a map, extend 
from 50° to 60° N. Latitude, and from 3° E. to 7° W. 
Longitude. 
Draw an indefinite straight line AB (Plate CCLXVII. 
Fig. 6.) for the parallel of 50°, and from a point C 
near the middle of the map, erect a endicular for 
the middle meridian, or that of 2° W. longitude. From 
C set off to D 10 equal parts, taken from a scale of an. 
convenient length, as inches, to denote degrees of lati- 
tude, and through D draw EF parallel to AB, for the 
parallel of 60°. Take from the ‘Table, p. 151, the frac- 
tion corresponding to 50°, which is .64279 or .643 near- 
ly, and it will be the length in parts of the same scale, 
in this case inches, to be set off from C towards A, and 
from C towards B for degrees of longitude. In like 
manner, take the fraction opposite to 60° in the Table, 
which is .5, and it will give the length to be set off from 
D to E.and from D to F. Then lines drawn through 
the divisions of CD, parallel to AB, will be parallels of 
latitude, and lines joining the corresponding divisions 
in AB and EF will be meridians. A scale of miles 
adapted to. the map, “may be constructed as formerly 
explained under Flamstead’s projection. 
Of the objects to be delineated on a map, or of the 
method. of representing them, it will not be necessary 
to say much, as such details must be familiar to all our 
readers. The great natural division of the globe, is into 
land and water. The subdivisions of the former are 
continents, or large tracts containing several kingdoms 
and states, as Europe, Asia, Africa, and America, and 
islands, or smaller tracts, wholly surrounded by water, 
as Britain. A tract of land, surrounded with water on 
all sides but one, whatever be its extent, is called a 
peninsula, as Spain and Africa; and the side by which 
it is united to other land, is called an isthmus, as the 
isthmus of Suez, which joins Africa to Asia.. A point 
of land running into the sea is called a cape, promonto- 
ry, or head-land. The subdivisions of the water are 
oceans, or those large collections which surround the 
continents, and which are usually reckoned five, viz. 
the Northern, Southern, Atlantic, Pacific, and Indian ; 
and seas, or those branches of the ocean which intersect 
the continents, as the Baltic and Mediterranean. When 
a narrow branch of the sea, or ocean, projects far into 
the land, it is called a gulf, as the Arabian Gulf, and a 
bays when its entrance is wider, as the Bay of Biscay. 
The canal that unites a gulf with a'sea or an ocean, is 
called a strait, as the Straits of Babelmandel and Gib- 
raltar, All these divisions are traced out on the map, 
by a crooked or waving line representing the coast, 
from which small parallel lines are drawn towards the 
water, of about a tenth, or sometimes two tenths of an 
inchin length. These lines, while they render the se- 
paration more distinct, have also the effect of making the 
sea appear to project from the surface of the map. In 
charts, or maps chiefly intended for representing 
coasts, harbours, &c. the direction of the parallel lines 
is reversed, which gives to the land the appearance of 
projecting. The latter seems to be the most natural re- 
presentation. In modern maps, the parallel lines are 
generally drawn quite across the sea, from coast to 
coast, by which the division of land and water is ren- 
dered still more distinct. In representing mountains, 
. geographers formerly employed vertical sections, as A 
(Fig. 7.) but of late, the bird's eye view is more fre- 
quently used. This consists in small waving lines, as 
GEOGRAPHY. 
B, diverging from a point, the point representing the 
stuseatiy aca the sas the declivity of the mountain. 
Rivers are represented by waving lines, roads by crook- 
ed lines, either single or double, and towns by small 
circles, varying in magnitude, according to the size of 
the towns themselves, and the scale of the map. 
As the reduction of maps from one scale to another, 
is frequently a problem of importance in practical geo. 
pers we shall point out a method of making such 
a reduction, which, if not extremely accurate, deserves 
to be noticed for its simplicity. 
Let ABCD (Fig. 8.) be a given 
similar figure of a reduced size, u 
red to lay down points correspon 
to trace aline corresponding to the line LMN. Di- 
vide ABCD into any number of equal squares, or paral- 
lelograms, by straight lines parallel to AB and AD, 
and divide abcd also into the same number, by lines 
el to ab, ad; then, by comparing the co 
andabcda 
ing parallelograms, points e, f; g, 4 may easily be found 
occupying very nearly the 
the points E; F, G, 
same positions in a6 cd, that 
LMN. This method may be advantageously employ- 
ed for filling up the details of a map, after the princi- 
pal points have been determined by some more accu«: 
rate method, For the use of the pentagraph, in redu-_ 
cing maps, see the article Drawine InstRUMENTS, Vol. 
viii. p. 129. See, for the construction of maps, Precis 
dela Geographie Universelle, par M. Malte Brun, tom. 
ii. Pare lee pe sur da Rocapention des Pron 
Geographiques, par M. Henry, Paris 1810; a, 
Principii i Geografia, Verona 1789 ; Playfair, Outlines 
of Natural Philosophy, vol. ii. p. 66, &c. 
Sect. V. On the Determination se Longitudes and 
Latitudes of Places on the Earth's Surface. 
In the preceding Sections, we have pointed out the 
various methods which may be employed in deli 
the whole or a part of the earth’s surface, either upon a 
sphere or upon a plane. » Before a map, however, ‘can 
be completed, it is necessary to have exact position 
of various places in reference to the equator; and to 
some fixed meridian ; and hence the determination of' 
the longitudes and latitudes of places by astronomical, 
be pair me or chronometrical observations, is one 
of the most important operations in geo: bys <>. 
We have death soaked out in ee AstRo- 
nomy, the method of making these observations by the 
aid of the planets or the fixed stars ; and in our articles 
NavicaTrion, SuRVEYING, and TiMEKrEPER, we shalk 
have occasion to consider the method of determini 
longitudes and latitudes by trigonometricai instruments, 
and by chronometers. e shall, therefore, conclude. 
this article with a Table of geographical positions, ex 
hibiting the latitude of the principal points on the earth’s 
surface, and their longitudes, or difference of meridians, 
in relation to the observatory of Greenwich. 
This Fable, which is by far the most correct that has 
ever been published, has been taken principally.from the 
Connoissance des Tems for 1816, and contains the results 
of the best observations which have been made by astro= 
nomers and navigators, and by those eminent individu~ 
als who have from time to time been employed in mea-« 
suring degrees of the meridian. We have corrected: 
many of the positions, and have added more than two 
hundred new places in England and Scotland, from the 
accurate observations of Colonel Mudge and \Captain. 
Coleby. No place is. inserted, unless its position has: 
been actually determined either by astronomical, trigo«. 
pometrical, or chronometrical observation. 
‘thich it is requi- reducing 
ing to B, F,G,H, and maps. 
do in ABCD, andalinelmn . 
may also be traced, differing little in its direction from 
Mather 
phy. 
Method 0 
Fig. 8. 
