sphere the Flinders bar has blue polarity in its upper end and red polarity in its lower end; at the magnetic equator it loses its. magnetism, but when brought into the southern hemisphere it again acquires magnetism, its upper end then possessing red polarity and the lower end blue polarity. The object, therefore, to be attained by the Flinders bar is to place it within the binnacle in such a position that the gradual change of its magnetism, caused by the change in latitude, will counterbalance the effect of the likewise varying magnetism of the vertical iron of the ship.

When on the equator it would be easy to compensate the semicircular deviation by magnets, and then, on proceeding north or south, compensate the error afterwards appearing on easterly and westerly courses by means of a Flinders bar.

NOTE. According to experiments recently made by the Superintendent of Compasses, and published in the Report of Chief of the Bureau of Equipments, Navy Department, Washington, D. C. (1900), it is more convenient and furnishes better results to correct the quadrantal deviation before compensating for B and C of the semicircular deviation; it saves time and labor, since, if B and C are compensated before D, allowance has to be made for an approximate or accurate D, which always causes much unnecessary shifting of magnets.

97. Heeling Error.-The deviation hitherto considered has been for vessels in an upright position. But when, from some cause, the ship has a list to either side, a new error is

created that is generally known as the heeling error. The principal cause of this error may be illustrated by Fig. 26. When the ship from the pressure of wind, shifting of cargo, or unequal trimming of bunker coals, heels over, all

horizontal iron, such as the deck beams m n, tend to assume a vertical position, and in doing so receive magnetism by induction from the earth. Thus, for a ship in the northern hemisphere the upper end of the beams, whether heeling to port or to starboard, will acquire blue polarity and the lower end red polarity, as shown in the figure. In the southern hemisphere these conditions are reversed. As a consequence, the north end of the compass needle will be attracted by the upper ends of the beams in north magnetic latitudes and repelled in the south magnetic latitudes, and the amount of this error will evidently depend on the amount of heeling.

98. From the illustration given, it is evident that when the ship's head is on magnetic north or south the disturbing force acts in a direction perpendicular to the needle, and, hence, the greatest amount of heeling error is caused when the ship is heading on, or near the direction of the magnetic meridian. On the other hand, when the ship's head is on magnetic east or west the disturbing force from heeling, still acting perpendicular to the fore-and-aft center line of the ship, will have a tendency to bring the needle in the direction of the magnetic meridian and, hence, but very slightly affects its direction; the only effect of heeling in this case will be an increase or decrease of the directing power of the magnetic needle. This explains the reason why the heeling error is greatest on northerly and southerly courses and least on easterly and westerly courses.

99. To Compensate the Heeling Error.-The compensation of the heeling error may be accomplished by several methods, the best and simplest being that of placing a magnet rb vertically below the center of the compass bowl, as shown in Fig. 26. Before compensating, the ship is swung into a north-and-south direction and heeled over at least 5 degrees, for instance, to starboard, as in Fig. 26 (x). If in this position the compass north is deflected toward the uppermost or windward side (as is usually the case), the compensating magnet is placed with its red pole uppermost

and at a distance from the compass bowl that is determined by raising or lowering the magnet until the compass points. correctly. In the very exceptional case of the needle being deflected toward the lower or leeward side, the blue pole of the magnet is placed uppermost.

100. It must be borne in mind that the compensation for heeling error is, theoretically speaking, good only for the latitude in which it is made, and it therefore becomes a

FIG. 27

necessity to renew its compensation when the ship has changed her latitude considerably. At the magnetic equator the error is at its minimum, and when entering the southern hemisphere it again increases in amount, although of a different character; in southern magnetic latitudes, therefore, the vertical magnet will have to be reversed.

101. Fig. 27 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; ab is the tube containing the magnet for heeling error; c and d are the soft-iron correctors for the quadrantal deviation, and e is an instrument called the clinometer, which shows the amount of heel; although not shown in cut, the binnacle is also fitted with a receptacle for the Flinders bar. Many other styles of compensating binnacles, more or less elaborately fitted out in regard to compensation, are at present to be found on the market and in use on all classes of vessels.

NOTE. As stated before, the object of the preceding articles on compass compensation is simply to give the student an insight of how the several errors of the compass are adjusted. 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 an effort to bring it within manageable limits.

HOW TO DETERMINE AND TABULATE THE

DEVIATION

102. After having minimized the effect of the several disturbing forces 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.

3. By observations of amplitudes and azimuths.

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.

103. 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 their proper position; anchors weighed; and pieces of iron and steel, such as chains, bolts, tools, etc., should be removed and not allowed to remain near the compass; if in a steamer, steam should be up. The ship is now moored to a buoy (preferably to a mooring made of timbers driven down vertically in the bottom and lashed together), as shown in Fig. 28, and swung around by means of a tug. 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 circumstance should the tugboat be allowed to lie alongside the ship while swinging. 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. 28) will make no sensible difference in the real bearing from the object O to the central point B. The distance c O must depend on the range the ship takes when swung; if she be at anchor in a tideway, 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.

104. 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