255 
NAVIGATION. 
A NAVIGATION, the art of conducting a ship 
from one port to another. The main end of all 
practical navigation is, to conduct the ship in 
safety to her destined port ; and for this purpose 
it is of the utmost consequence to knbw in what 
particular part of the surface of the globe she is 
at any particular time. This can only be done 
by having an accurate map of the sea-coasts of 
all the countries of the world, and, by tracing 
out the ship’s progress along the map, to know 
at what time she approaches the desired haven, 
or how she is to direct her course in order to 
reach it. It is therefore a matter of great im- 
portance for navigators to be furnished with 
maps, or charts, as they are called, not only 
very accurate in themselves, but such as are 
capable of having the ship’s course easily traced 
upon them, without the trouble of laborious cal- 
culations, which are apt to create mistakes. 
The navigator should have a perfect knowledge 
of the figure and motion of the earth ; the vari- 
ous real and Imaginary lines upon it, so as to be 
able to ascertain the distance and situation of 
places with respect to one another. He should 
also be acquainted with the several instruments 
employed in measuring the ship’s way ; such as 
the log, half-minute glass ; quadrant to take the 
altitude of the sun and stars ; compass to repre- 
sent the sensible horizon . and azimuth compass 
to take the azimuth arid amplitude of the sun, in 
order to know the variation of the magnetic 
needle. He should have an accurate knowledge 
of maps and charts of the lands and seas, toge- 
ther with the depth of water, the times and set- 
t'ng'in of the tides upon the coasts that he may 
have occasion to visit ; also the currents.; of the 
mould and trim of the ship, and the sail she 
bears, that so a due allowance may be made for 
lee-way. By the help of these, he may at all 
times know the place the ship is in, which way 
he must steer, and how far he has to run to gam 
his. intended port. 
The names of the two great divisions of naviga- 
tion are taken merely from the kind of charts 
made use of. Plane sailing is that in which the 
plane chart is made use of; and Mercator’s 
sailing, or globular sailing, is that in which 
Mercator’s chart is used. In both these methods, 
it is easy to find the ship’s place with as great 
exactness as the chart will allow, either by the 
solution of a case in plane trigonometry, or by 
geometrical construction. 
Qf Plane sailing. As a necessary preliminary 
to our understanding this method of navigation, 
we shall here give the construction of the plane 
chart.. 
This chart supposes the earth to be a plane, 
1 . 
and the meridians parallel to one another ; and 
likewise the parallels of latitude at equal dist- 
ances from one another, as they really are upon 
the globe. Though this method is in itself evi- 
dently false; vet, in a short run, and especially 
near the equator, an account of the ship’s way 
may be kept by it tolerably well. 
Having determined the limits of the chart, 
that is, how many degrees of latitude and lon- 
gitude, or meridional distance (they being in 
this chart the same), it is to contain : suppose 
from the.lat. of 20° N. to the lat. of 71° N., and 
from the longitude of London in 0 deg. to the 
long, of 50° W.; then choose a scale of equal 
parts, by which the chart may be contained 
within the size of a sheet of paper on which it 
is intended to be drawn. 
'Make a parallelogram ABCD (Plate Naviga- 
tion. fig. 1) . the length of which AB from north 
to south shall eontnm .71 degrees, the difference 
of latitude between the limits' of 20° and 71°; 
and the breadth* AD from east to west shall 
contain the proposed 50 degrees of longitude, 
the degrees being taken from the said scale, 
and this parallelogram will be the boundaries 
of the chart. 
About tile boundaries of the chart make 
scales containing the degrees, halves, and quar- 
ters of degrees (if the scale is large enough) ; 
drawing lines across the chart through every 5 
or 10 degrees ; let the degrees of latitude and 
longitude have their respective numbers an- 
nexed, and the sheet is then fitted to receive the 
places intended to be delineated thereon. 
On a straight slip of pasteboard, or stiff paper, 
let the scale of the degrees and parts of degrees 
of longitude, in the line AD, be laid close to the 
edge ; and the divisions numbered from the right 
hand towards the left, being all west longitude, 
Seek in a geographical table for the latitudes 
and longitudes of the places contained within 
the proposed limits ; and let them be written out 
in the order in which they increase in latitude. 
Then, to lay down any place, lay the edge 
of the pasteboard scale to the divisions on each 
side the chart, shewing the latitude of the place; 
so that the beginning of its divisions falls on the 
right-hand border AB ; and against the division 
shewing the longitude of the given place make 
a point, and this gives the position of the place 
proposed ; and in like manner are all the other 
places to be laid down. 
Draw waving lines from one point to the 
other, where the coast is contiguous, and thus 
the representation of the lands within the pro- 
posed limits will be delineated. 
Write the names to the respective parts, and 
in some convenient place insert a compass, and 
the chart will be completed. 
2. The angle formed by the meridian and 
rhumb that a ship sails upon, is called, as we 
have said, the ship’s course. Thus, if a ship 
sails on the N.N.E. rhumb, then her course will 
be 22° 3(y ; and so of others, as is manifest from 
the following table of the angles which every 
point of the compass makes with the meridian. 
North. 
South. 
Points. 
D. M. 
North. 
South. 
_r 
2.49 
i 
a 
5.87 
8.26 
N. by E. 
S. by E. 
i 
1115 
N. by W. 
S. by W. 
i i 
14. 4 
1 i 
1G 52 
1 1 
19.41 
N. N. E. 
S. S. E. 
2 
22.30 
N. N. W. 
S. S. W. 
2 i 
25.19 
2 i 
28. 7 
2 | 
30.56 
N E. by N. 
S. E. by S. 
s 
33.45 
N.W. by N. 
S. W. by S„ 
s i 
36.34 
3 h 
39.22 
o 4 
u 4 
42.11 
N. E. 
S. E. 
4 
45. 0 
N. W. 
s. w. 
4 i 
47.49 
4 i 
50.37 
4 | 
53. 2G 
N. E. by E. 
S. E. by E. 
5 
56.15 
N.W. by Wj 
S.W. by W. 
5 f 
59. 4 
5 k 
61.52 
5 ! 
64.42 
E. N.E. 
E. S. E. 
G 
67.30 
W. N. W, 
W. S. W. 
6 i 
70.19 
8 i 
73. 7 
« 1 
75.56 
E. by N. 
E. by S. 
78.45 
W. by N. 
W. by S. 
7 
81.34 
7 — 
84.22 
7 I . 
87.11 
, East. 
8 
90. 0 
West. 
3. The distance between two places lying on 
the same parallel counted in miles of the equa- 
tor, or the distance of one place from the meri- 
dian of another counted asabove on the parallel 
passing over that place, Is called meridional dis- 
tance ; which, in plane sailing, goes under the 
name of departure. 
4. Let A (fig. 2), denote a certain point on 
the earth’s surface, AC its meridian, and AD 
the parallel of latitude passing through it ; and 
suppose a ship to sail from A on the N. N.E. 
rhumb till -she arrives at B ; and through Bdraw 
the meridian BD, (which, according to the prin- 
ciples of plane sailing, must be parallel to CA,) 
and the parallel of latitude BC; then the length 
of AB, viz. how far the ship lias sailed upon the 
N. N.E. rhumb, is called her distance ; AC or BD 
will he her difference of latitude, or northing 
CB will be her departure, or easting; and the 
angle CAB will be the course. Hence it is 
plain, that the distance sailed will always be, 
greater than either the difference of latitude or 
! departure; it being the hypothenuse of a right- 
I angled triangle, whereof the other two are the 
I legs; except the ship sails either on a meridian 
or a parallel of latitude : for if the ship sails on 
| a meridian, then it is plain, that her distance: 
I will be just equal to her difference of latitude, 
l and she will have no departure; but if she sails 
! on a parallel, then her distance will be the same 
j with her departure, and she will have no differ 
! ence of latitude. It is evident also from the 
i figure, that if the course is less than 4 points 
; or 45 degrees, its complement, viz. the other 
! oblique angle, will be greater than 45 degrees, 
| and so the difference of latitude will be greater 
j than the departure ; but if the course is greater 
than 4 points, then the difference pf latitude 
1 1 
