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with the ship heeling at least 5° with her head in the direction that was north when in an upright position.
In reference to steamers plying on the Great Lakes, the heeling error is of no particular importance.
101. Fig. 26 represents a compensating binnacle with the door open, showing the arrangement of magnets for
compensating the different errors of the compass. The ends of the magnets are painted red and blue, according to their polarity; a b is the tube containing the magnet for the heeling error, c and d are the soft-iron correctors for the quadrantal deviations, and e is an instrument called the clinometer, which shows the amount of heel. Although not shown in the cut, the binnacle is also fitted with a receptacle for the Flinders bar. Many other styles
compensating binnacles, more or less elaborately fitted out in regard to compensation, are at the present day to be found on the market and in use on all classes of vessels.
Concluding Remarks. – As stated before, the object of the preceding articles on compass compensation is simply to give the student an insight into the methods of adjusting the several errors of the compass. In practice, the adjustment may be more elaborate or more simple, according to requirements. As a whole, it should be borne in mind that the object of adjustment is not an attempt at strict accuracy in removing the deviation, but rather to bring it within manageable limits.
DETERMINING AND TABULATING THE
1. After having minimized the effect of the several disturbing forces surrounding the compass, there still remains a certain amount of deviation that must be known and tabulated. The principal methods used for ascertaining this deviation are as follows:
1. By the bearing of a distant object, the correct magnetic bearing of which is known.
2. By reciprocal bearings.
The first and second of these methods are used when the ship is in port, and consist mainly of a process known as swinging the ship.
2. First Method. - When about to determine the deviation, the ship should, in all respects, be ready for sea; stores and weights on board properly disposed; boats hoisted, and davits swung into proper positions; anchors weighed; if a steamer, steam should be up. All pieces of iron and steel, such as chains, bolts, tools, etc., should be removed and not allowed to remain near the compass. The ship is then moored to a buoy (preferably to a mooring made of timbers driven vertically in the bottom of the harbor and lashed together), as shown in Fig. 1, and swung around by means of a tugboat. In swinging the ship, the tugboat T used for this purpose should be given enough hawser so as not to influence the compass by its iron funnel or smokestack. Under no circumstances should the tugboat be For notice of copyright, see page immediately following the title page
allowed to lie alongside the ship while the swinging is in progress. Preparatory to swinging, a well-defined object, or mark, on land should be selected, so far distant that the diameter of the space through which the ship is swung (see dotted circle, Fig. 1) will make no sensible difference in the real bearing from the object to the central point B. The distance c O must depend on the range the ship takes when swung; if she is at anchor in a bay or harbor, 5 to 7 miles
will be sufficient (by some authorities 10 miles); if swung in a dock, at mooring, 1 or 2 miles will suffice, but the distance should never be less than a mile.
3. The next thing to be determined is the correct magnetic bearing of the selected object from the ship. This can be done in several ways: by taking the mean of the compass bearings of the distant object, which gives its correct magnetic bearing; by comparing a compass with the standard compass and placing it on shore, in a place free from local attraction, on the line of bearing between the standard compass and the distant object; and by taking the bearing 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, the boat steady, and the bearing carefully taken.
This method of using a boat is preferable to bringing the compass on shore, where it might be subjected to unknown local magnetic influences. The method by taking the mean of the compass bearings, however, is to be preferred.
4. The magnetic bearing of the object having been determined, the ship is gradually swung around 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
Mag. points, will show the error on each of
Comp. these points; or, in other words, the deviation of the standard compass according to the direction in which the ship's head was placed.
5. To Name the Deviation. – The rule to be remembered when naming the deviation is as follows:
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. 2, represents the north end 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, or right, side of the magnetic north. If the north end of the needle had been deflected to n', the deviation would be westerly, according to the foregoing rule.
6. Tabulation of Bearings. - The following arrangement of the data in tabular form is convenient to use when recording the results of swinging a ship by this method:
CORRECT MAGNETIC BEARING OF DISTANT OBJECT FROM
SHIP = N 63° W; DISTANCE 10 MILES
SSE S by E
N 59.8° W 3.2° W
3.2°E 0.1° E 3.0° W 6.5o W 9.7° W 13.0° W 16.2° W 19.2° W 21.2° W 23.3° W 24.0° W 23.6° W 22.0° W 19.0° W 14.8° W 9.2° W
Sum - 1,223.4 (For 16 Points.) Sum 786.6 (For 16 Points.)
1,223.4 + 786.6 Correct magnetic bearing
N 62.9° W, or N 63o W. 32
7. 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