24 G 
FOREST AND STREAM 
Feb. 24, 1912 
Simple Navigation. 
BY A. G. GOLDSMITH. 
(Concluded from page 216.) 
To name the deviation correctly, either east or 
west, let the observer imagine himself standing 
at the center of his compass looking toward the 
bearings of the objects in question, then— 
If the magnetic bearing is to the right of the 
observed bearing, name the deviation east. 
If the magnetic bearing is to the left of the 
observed bearing, name the deviation West. 
EXAMPLE—yacht's HEAD BY COMPASS, NORTH. 
Magnetic bearing of A and B. N. 30 W. 
Observed bearing of A and B. N. 27 W. 
Deviation . 3 W. 
EXAMPLE—yacht's HEAD BY COMPASS, WEST. 
Magnetic bearing of C and D. N. 88 W. 
Observed bearing of C and D. S. 88 W. 
176 
180 
Deviation . 4 E. 
In this case, one hearing being north and west, 
while the other was south and west, it was neces¬ 
sary to add the two together and subtract their 
sum from 180 (the total number of degrees in 
both quadrants) in order to find the amount of 
the deviation. It is named of course in the usual 
way. 
In many ports there are marks specially put 
up, either on the dock walls, or in the shape of 
buoys moored in the harbor or roadstead, whose 
correct magnetic bearing, taken in line with some 
building, tower or other leading mark, are pub¬ 
lished in the port authorities’ book of rules, also 
in Brown’s Nautical Almanac. These are of 
considerable use to those who wish to test their 
compasses for deviation before proceeding on a 
cruise, as it saves them the trouble of finding 
reliable marks elsewhere. 
There are of course other methods for ascer¬ 
taining the deviation of compasses, applicable to 
cases where it is impossible, or inconvenient to 
obtain bearings of objects ashore or afloat, but 
as these methods involve stellar or solar observa¬ 
tions, I propose to deal with them later. 
Given a reliable compass, then, whose deviation 
has been obtained by the foregoing means, the 
amateur navigator must turn his attention to 
choosing a stock of large scale charts of the 
coasts off which he intends cruising. 
Old charts should not be relied upon, espe¬ 
cially in narrow waters, as the position and char¬ 
acter of the buoys and lights marking the reefs 
and other dangerous places, to say nothing of 
the depths of the water on the shoals and bars, 
are constantly changing. The average price of 
a perfectly- up-to-date chart is iis., so even if 
half a dozen are required, the outlay is not a very 
considerable one. 
For laying off bearings on the charts, there is 
nothing to equal an ordinary pair of parallel 
rulers, which, being engraved on the front edge 
and ends with marks representing the degrees 
contained in two right angles, enable the navi¬ 
gator to dispense with the awkward little com¬ 
passes drawn in various places on the chart, and 
lay all his bearings down from one of the 
meridians of longitude. 
One hears a lot said about course protractors, 
station pointers, etc., but very few professional 
navigators ever make use of them outside the 
navy, for they are quite unnecessary, and of no 
particular advantage. Another point against them 
is that they are by no means inexpensive (espe¬ 
cially the latter), so the amateur navigator will 
do well to dispense with them altogether, and 
lay his money out in more useful directions. 
A somewhat expensive but exceedingly useful 
little instrument for taking compass bearings is 
that known as Sir- W. Thomson’s Azimuth 
Mirror. Despite its somewhat puzzling wame, 
it is exceedingly easy to use, being simply a httle 
prism, which when p’aced on the glass Cover of 
the compass bowl enables the navigator to reflect 
objects on the horizon or elsewhere directly on 
N 
DIAGRAM 3 . 
Compass as found marked on a true chart. 
DIARGAM 5. 
All courses and bearings in black quadrants are right- 
handed; those in -white quadrants are left-handed. 
to the rim of the compass card, from which their 
bearings can be determined to a fraction of a 
degree at a glance. Most yacht’s compasses are 
fitted with sight vanes, but none of these are 
nearly as easy to manipulate, or yet so accurate 
as this simple little Azimuth Mirror:’ 
Turning now to the subject of charts, one fre¬ 
quently hears navigators talking about “working 
from true charts” - or “working from magnetic 
charts ” 
This may be somewhat muddling to the begin¬ 
ner, and is due to the fact that the majority of 
coasting “skippers” will still favor the old method 
of using the compasses drawn in certain places 
on the charts for the purpose of reading off 
bearings and laying down courses. The modern 
navigator never by any chance pays the least at¬ 
tention to them, except to read from them the 
amount of the local variation. 
'What is known as the “true chart” is one in 
which the compasses inscribed thereon—regard¬ 
less of the existing variation—have been placed 
with their north points exactly over one of the 
meridians of longitude. On the other hand, mag¬ 
netic charts are those in which the compasses are 
drawn with their north points either to the east 
or west of the meridian, according to the amount 
of the local variation. There is, however, no 
need to trouble with them at all if a pair of 
Field’s rulers are employed. Personally, I have 
seen more dangerous mistakes made through 
using them, to say nothing of general inaccuracy 
in the laying down of courses and bearings, etc., 
owing to their small diameter and indistinctly 
marked points and degrees, than from any other 
means whatsoever. 
I have of course met many men who invariably 
did all their calculations in points and reduced 
all bearings to magnetic ones, using an ordinary 
pair of rules and these compasses engraved on 
the charts. I have also had the pleasure of 
pointing out to many of them the all-round ad¬ 
vantages of using Field’s rulers and working in 
degrees. In the majority of cases they immedi¬ 
ately adopted this latter method, finding it as 
superior to the other as the metric system is to 
our British “rod, ell, link, pole, furlong, etc.,” 
measures. 
The following example will clearly demonstrate 
the simplicity of this system of chart work : 
The navigator wishes to set a course from his 
position A to pass five miles south of a point C. 
Placing one edge of his rulers straight along a 
meridian, he works them across until they are 
directly over C, drawing a line from it when he 
had done so. This line bears due south from C. 
Then he takes his dividers, and measuring five 
miles on them from the latitude column at the 
side, proceeds to mark off the required d’stance 
on this line. This spot we will call B. Placing 
the front edge of the rulers, upon which the de¬ 
grees are cut, so that it passes exactly through 
the points A and B, he proceeds as in the case 
of finding the magnetic bearing of the two ob¬ 
jects shown in Diagrams i and 2. 
Having found the true course, he then turns 
it into the corresponding compass course by the 
following rules: 
(1) To turn a true course into a compass 
course— 
If variation or deviation is east, allow it to 
the left of true course. 
If variation or deviation is west, allow it the 
right of true course. 
The result is the course to be steered by com¬ 
pass. 
(2) To turn a compass into a true course— 
If variation or deviation is east, allbw it to 
the right of compass course. 
If variation or deviation is west, allow it to 
the left of compass course. 
The result being the true course. 
In this case we will suppose that the true 
course from A to B is N. 70 E., the variation we 
learn from the chart is 13 degrees west, and the 
deviation, we note, on referring to our compass 
deviation card, for N. 83 E. 
(N. B.—Always apply the variation to the true 
course before referring to the deviation card, as 
it gives greater accuracy) is two degrees east. 
He then proceeds in this fashion : 
Rule I. 
True course (rieht hand). N. 70 E. 
Variation (right hand) . 13 W. 
Rule I. 
Magnetic course (added because both 
are right hand). N. 83 E. 
Deviation (left hand).. 2 E. 
Compass course (subtracted because 
one is right hand and the other 
left) . N. 81 E. 
