Comp.Bearing N59.8W- Mag. object is determined-in this case N 63° W - the ship's head is swung, for example, north, according to the standard compass. Now, at the same instant that the ship is heading exactly north, a bearing of the object is taken and found to be N 59.8° W. According to the rule of Art. 5, the deviation is then 63° - 59.8° = 3.2° west, because the correct magnetic bearing N 63° W lies to the left of the compass bearing N 59.8° W, as shown in Fig. 3. Comp Bearing N83:3W W E FIG. 3 Again, when the ship's head is toward the east, according to the standard compass, the bearing of the object is N 83.3° W (see Table I); the deviation for this (east) point is consequently 83.3° 63° 20.3° east, because the correct magnetic bearing lies to the right of the compass bearing. In ordinary practice the finding of the deviation to the nearest degree or half degree is quite sufficient. = 8. If the correct magnetic bearing of the object had not been known beforehand, it could be readily obtained by simply adding all compass bearings and dividing the sum by 32; in other words, the mean of all bearings will give the correct magnetic bearing of the object, as shown at foot of Table I. 9. Second Method. - When no suitable object on land is visible from the ship at the requisite distance mentioned in Art. 2, the deviation is determined by what is known as simultaneous reciprocal bearings. This method consists of a compass being brought on shore and placed on a tripod in a carefully selected spot, where it will be free from the magnetic influence of any iron and where its location can be distinctly seen from the standard compass on board. As the ship is swung around, with her head successively upon each of the 32 points of the standard compass, simultaneous observations, or bearings, are taken by the observer stationed at each compass, according to some prearranged signals. 10. In order to obtain satisfactory results, the shore compass should not be placed at so great a distance from the ship that the observer on board cannot distinctly see it without the aid of glasses; likewise, the standard compass, or the observer standing directly behind it, should be clearly visible to the shore observer. The bearings must be strictly simultaneous and the time noted at each observation; if convenient, the shore observer, or his assistant, should chalk each observation on a blackboard, so that it may be read at once from the ship. Either before or after the observations have been made, the standard compass should be brought on shore for the purpose of comparing it with the shore compass by means of the bearing of some distant object, the difference, if any, to be recorded and properly applied. 11. To obtain the deviation resulting from observations by this method, each of the bearings taken by the shore compass must be reversed and considered as the correct magnetic bearing when applied according to the rule of Art. 5. 12. It will be convenient to tabulate the bearings and results in a form similar to the following: And so on for all points of the compass. From the above form, deviation tables are prepared. Deviation tables for use in navigating a ship need not necessarily contain or give the deviation closer than the nearest half degree, fractions below and above .5° being omitted. 13. The following form of deviation table is the most common in use on shipboard: Column 3 of this table is obtained by applying the deviation in column 2 to the points of the compass in column 1; hence, column 3 indicates what course should be steered by the standard compass in order to make good any required correct magnetic course. Thus, if it is required to steer N NEE correct magnetic course, look in column 3 and find N 30° E (which agrees very nearly with N NE & E); the corresponding line in column 1 will then indicate the course to be steered-in this case N E by N. 14. Correction of Bearings. — When a bearing is corrected for deviation, remember not to correct it for the deviation on the bearing, but for the deviation due to the direction of the ship's head at the moment the bearing was taken. This is very important. EXAMPLE.-The bearing of a distant object by the standard compass is ESEE; the ship is heading N W by W. Required the correct magnetic bearing. SOLUTION.- ESEE 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 in Fig. 4. 15. Third Method. - The method of determining the deviation by observations of amplitude and azimuth is used when the ship is at sea, and will be described in Lake Navigation, Part 4. W Mag. N S E S73 E the 16. Limitations of 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 were made, and cannot be applied to other compasses in the ship; furthermore, the deviation is not applicable to the compass if it is removed and placed in some other part of the ship. It is evident that, while swinging a ship, the deviation of all the compasses may be found if the direction of the ship's head, as indicated by each, is noted and tabulated. FIG. 4 17. 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 that compass. For instance, if you want to steer N E, the ship's head is put in a N E direction 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 N E. 18. When the deviations are small, as is the case in ships where compasses are carefully adjusted, it is sufficient to determine the deviation for the eight principal points deviations for intermediate positions of the ship's head may then be found by means of a diagram. only; are a WEST EAST NORTH (5°E)* -WSW -(6°W) WEST WNW (6:5W)- -NW NNW NORTH SE * 5 10 15 Scale: I Division= 1° of Deviation FIG. 5 ئسية 19. Deviation Diagrams. - There several kinds of deviation diagrams in use; one of the simplest is shown in Fig. 5. To construct this diagram, take a sheet of ruled paper and mark the points of the compass on 32 consecutive lines. From a vertical line ab, drawn through the center of the paper, lay off, on the horizontal lines, the deviations for the eight principal points on any desired scale, and draw a curve through the points thus obtained. The distance from the vertical line to the curve, according to the given scale, will give the deviation with considerable accuracy for any desired direction of the ship's head. |
