of the distant object from that point by the shore compass. If the selected object is too far off to bring the compass on shore, the required bearing may be obtained from a compass placed in a ship's boat r at a suitable distance from the ship on the line of bearing. In this case the water must be smooth and the boat steady, and the bearing must be carefully taken. This method of using a boat is preferable to placing the compass on shore, where it may be subjected to unknown local magnetic influences. The first method, by taking the mean of the magnetic bearings, however, is to be preferred. 105. The magnetic bearing of the object being determined, the ship is gradually swung round so as to bring her head successively upon each of the 32 points of the standard compass, steadying at each. The difference between the correct magnetic bearing of the object and the successive bearings, as observed with the compass on board when the ship's head is on the several points, will show the error on each of these points, or, in other words, the deviation. of the standard compass according to the direction in which the ship's head was placed. 106. To Name the Deviation. The rule to be remembered when naming the deviations is as follows: Comp W n Mag. N FIG. 29 Comp. n Deviation is east when the north point of the compass needle is deflected to the east, or right, side of the magnetic north; and west when deflected to the west, or left side, of the magnetic north; or, if the correct magnetic bearing lies to the right of the compass bearing, the deviation is easterly; if to the left, the deviation is westerly. Thus, if n, Fig. 29, represents the north point of the compass needle and N the correct magnetic north, the angle NCn is the deviation, and is named easterly because n falls to the east of the magnetic north. If the north point of the needle had been deflected to n', the deviation would be westerly, according to the foregoing rule. 107. The following arrangement of the data in tabular form is convenient to use when recording the result of swinging a ship by this method. TABLE III CORRECT MAGNETIC BEARING OF DISTANT OBJECT FROM SHIP=N 63° W; DISTANCE=10 MILES 108. From this table the student will readily understand the procedure of finding the deviation by swinging the ship. After the correct magnetic bearing of the selected distant object is determined-in this case N 63° W-the ship's head is swung, for example, north, according to the standard com Comp Bearing N 59.8W - Mag. N pass. Now, at the same moment the ship's head is on the north point, a bearing of the object is taken and found to be N 59.8° W. According to rule, Art. 106, 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. Again, when the ship's head is toward the east, the bearing of the object is N 83.3° W, the deviation for this (east) point is, consequently, 83.3° -63°20.3° east, because the correct magnetic bearing lies to the CompBearing N83:3W W FIG. 30 right of the compass bearing. In ordinary practice the nearest degree of deviation is quite sufficient. 109. If the correct magnetic bearing of the object had not been known beforehand, it could be easily obtained by simply adding all 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 III. 110. Second Method.-When there is no suitable object on land visible from the ship at the requisite distance mentioned in Art. 103, the deviation is determined by reciprocal bearings. This method consists of placing a compass on shore at some well-selected spot where it will be free from the influence of any iron and where it 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 bearings are taken by the observers stationed at each instrument according to some prearranged signal. 111. 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 and compared with the shore compass-by means of the bearing of some distant object; the difference, if any, to be recorded and properly applied. 112. 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 rule of Art. 106. 113. 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. form, deviation tables are prepared. From the above Deviation tables for use in navigating a ship, however, need not necessarily contain more than the nearest halfdegree, any fraction below and above 0.5° being omitted. 114. The following form of deviation table is the most common in use on shipboard. TABLE V DEVIATION TABLE OF THE STANDARD COMPASS. Column 3 of the above 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 to make good any required correct magnetic course. Thus, if it is required to steer NNEE correct magnetic course, look to column 3 and find N 30° E (which agrees very nearly with N N EE); the corresponding line in column 1 will then indicate the course to be steered in this case NE by N. 115. Correction of Bearings.-When a bearing is corrected for deviation, remember, not to correct it for the |