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deviation on the bearing, but for the deviation due to the direction of the ship's head at the moment when the bearing was taken.
EXAMPLE.—The bearing of a distant object by the standard compass is E SEI E, the ship is heading NW by W. Required the correct magnetic bearing. SOLUTION.-By Table II, we find
ESE E corresponds to S 73° E
Correct magnetic bearing=S 90° E=east. Ans. Since the deviation in this case is westerly, or to the left, it is
added to the compass bearing, as shown Mag.
in Fig. 31.
117. Limitations of the Fig. 31
Deviation Table.—The student
must bear in mind that the deviation obtained by the methods described belong only to the compass by which the observations are made, and cannot be applied to other compasses in the ship; furthermore, the deviation is not applicable to the compass if placed in some other part of the ship. It is evident that, while swinging a ship, the deviation of all the compasses can be found if the direction of the ship's head, as indicated by each of them, is noted and tabulated.
118. In the merchant marine, however, it is usual to determine the deviation only for the compass selected as the standard, and then when about to set a course, to do so by the standard compass. For instance, if you wish to steer NE, the ship's head is put N E according to the standard compass, and if the steering compass does not agree, to
keep the course that it indicates when the standard shows correct NE.
119. When the deviations are small, as is the case in ships where the compasses are carefully adjusted, it is sufficient to determine the deviation for the 8 principal points only; the deviations for intermediate
WEST + EAST positions of the ship's
-(5'E)* head may then be found
-NNE by means of a diagram.
NE (11'E) 120. Deviation
ENE Diagrams.-There are several kinds of devia
-EAST tion diagrams; one of the simplest is shown in Fig. 32. To
SE-21* struct this diagram take a sheet of ruled paper and mark the points of the compass on 32 consecutive lines. From a
-(65W) 7-sw vertical line ab drawn through the center of the paper, lay off,
-16°W) the horizontal lines, the
WNW deviations for the 8 prin
(65W) cipal points on any desired scale and draw a through the
-NORTH5E) points thus obtained. The distance from the vertical line to the curve on the given scale will give the deviation with considerable accuracy for any desired direction of the ship's head. For instance, if it is required to construct a deviation diagram from the following deviations, the result will be as represented in Fig. 32.
Scale: 1 Division=l of Deviation
The deviation on any point can now be obtained by measuring the perpendicular distance between the curve and the desired point on the vertical line.
121. Illustration of Use of Diagram.—To find, for instance, the deviation for ESE } E proceed as follows: From c, representing the compass point E SE. E, draw a line perpendicular to a b. Then measure, with the scale, the distance cd, which, in this case, is equivalent to 63 divisions or 6.5°. Since the curve at that point lies to the east, the deviation is easterly; hence, deviation for ESEJE =6.5° east.
122. For larger deviations and greater accuracy a diagram, called, after its inventor, Napier's Diagram, is commonly used. Its construction is somewhat more complicated than the one just described, but it can be used with advantage in turning compass courses or bearings into magnetic courses or bearings, and vice versa.
Much reliance, however, should not be placed on tables of deviation, since the magnetic conditions change in different latitudes; beside, iron ships, as a rule, lose much of their magnetism in the first two years after being launched.
123. Proper Placing of Compass. — Considerable care should be bestowed on the selection of a suitable place for setting the standard compass on board a ship, remembering that no iron or steel of any kind must be placed near or permitted to remain within a certain distance of it. All vertical iron, such as stanchions, davits, etc. should, if possible, be at least 14 feet from the standard compass. It should be fixed in a position selected not for the convenience of the helmsman or builder, but in a place where the surrounding magnetic forces will least affect it, and where every facility exists for the examination of its error.
124. To Protect Compasses From Electrical Disturbances.-With the introduction of electricity on board ships a new form of compass disturbances has been created, inasmuch as the magnetism of the large electromagnets used in the dynamos and the electric currents in general may disturb a compass at a considerable distance. The committee of Lloyd's Register of British and Foreign Shipping has made the following suggestions in reference to protecting compasses from the influence of electricity on shipboard:
1. The dynamos and electric motors should be placed as far as possible from all compasses and at a distance of at least 30 feet from the standard compass.
2. That wires conducting electric currents should not come nearer than 16 feet to any compass, whereas wires conducting strong currents should be at a still greater distance.
3. That the compensating of compasses should be done when the dynamos are at rest, while the operations for determining the deviation should be performed when the dynamos are running.
125. Cautionary Remarks.-By a strict adherence to these precautions, arrangements, and practices, the compass may still, in a great measure, retain its place as the invaluable guide to the mariner in iron ships that it formerly was in wooden ships. Mere mechanical adjustment, however, should be regarded as only a temporary expedient. The careful navigator will therefore avail himself of every opportunity of determining his compass error by astronomical observations and compare the result so obtained with the tabulated deviation.
- True Course
CORRECTIONS OF COURSES 126. There are three kinds of courses; viz., compass course, magnetic course, and true course.
127. The compass course is the angle c OS, Fig. 33, that the ship's track O S makes with the direction of the magnetic needle. It may be affected True by variation and deviation and consequently must be corrected for both whenever the true course is to be obtained.
128. The magnetic course is the angle bOS that the ship's track makes with the magnetic meridian. This course is affected only by variation, the application of which converts it into a true course. It is evident that
FIG. 33 where there is no deviation the compass and magnetic courses will be identical.
129. The true course is the angle a OS that the ship's track makes with the true or geographical meridian, indicating true north and south. It is evident that at places on the earth's surface where neither deviation or variation exist the true course will be identical with the compass course.
130. In dealing with problems of correcting courses always remember that, since the compass is the representation of the visible horizon, the position of the observer is considered to be at the center of the compass card. Hence, when applying corrections, whether to the right or left the student must consider himself to be stationed at the center of the card looking in the direction of the course to be corrected. Thus one point to the right of ENE is E by N, and one point to the left of ENE is NE by E; similarly one point to the