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Consequently, observations and corrections must be made for each compass separately.

It is explained in the answer to Question 63 how this can be done for several compasses simultaneously.

Compensation good for all Latitudes. If the compensations are properly made when a ship is ready for sea, they should stand good for all latitudes, provided the sub-permanent magnetism does not change, for on this supposition the strength of the magnets and the coefficients of sub-permanent magnetism would always be in the same relation to each other, and no deviation would occur, although the directive power of the needle varies with change of latitude, whilst the horizontal force of transient induced magnetism and of the correctors would also remain in the same relation to each other.

Changes in Deviation.-But however carefully a compass may be compensated, changes may be expected with the lapse of time, and as these changes will be chiefly in the deviation due to sub-permanent magnetism, a readjustment of the magnets will be necessary from time to time. The effect of the vibration caused by the propeller, the buffeting of heavy seas, collisions, or stranding is to considerably modify the magnetic condition of the ship when she left the builder's hands.

Retained Magnetism.-Another troublesome cause of change of deviation is what is called retained magnetism; that is, if a ship remains for a considerable time in one position in dock or on the same course at sea for several days, the induced magnetism from the earth is retained for a certain length of time, and causes a deviation when the course is altered. In sailor's language, the ship tends to follow her last course. For example, a steamer in crossing the Atlantic, bound for Liverpool, lays on an easterly course for several days together, and when on rounding Tuskar her course is changed to the northward, the retained red polarity on the port side (and blue on the starboard side) causes an E. deviation, which tends to set her to the eastward of her proper course. To this cause is owing, no doubt, the fact that steamers from the westward, coming up Channel in thick weather, are apt to be found too near the Welsh coast, and steamers from the south, bound up the English Channel, too near the English coast.

ART. 127. Uncompensated Compasses.-An uncompensated compass having large deviations on some courses and very small deviations on others is very unsatisfactory for steering purposes, for two reasons:

(1) The deviation will be subject to greater changes, because when a ship approaches the magnetic equator the directive force of the needle increases, and therefore the coefficients of subpermanent magnetism, which are assumed to remain nearly

constant, will cause less deviation. On receding from the magnetic equator, the deviation would increase for the same

reasons.

(2) The directive tendency of the needle would be much increased when a strong blue magnetism in the ship is towards the north from the compass, and correspondingly decreased when red is towards the north. The consequence would be that in some directions a considerable change of course would show only a small change in the compass course, and in other directions a small change of course would appear a large one by compass.

Horizontal Iron and Uncompensated Compass.-The deviation caused by horizontal iron would not be subject to these changes, because the horizontal component of transient induced force would vary in the same ratio as the directive force of the needle.

Diminution of Directive Force.-In an ordinary iron ship, the directive force is rarely more than six-sevenths of what it would be in a wooden ship or on shore, and in armoured war-ships not more than three-fourths, according to Captain Creak, R.N., who has had very large experience of such matters. The chief cause of this is the horizontal iron, for in most cases it has a red polarity to the north of the compass. Sub-permanent magnetism weakens the directive force on some courses, but strengthens it on others, according as a red pole is north or south of the compass, and so the mean effect all round is nil. These remarks apply to uncompensated compasses.

The consequence of a diminution of directive force is greater deviations, for the forces which cause deviation are acting on a weakened compass.

The object aimed at by compensating a compass is to secure a uniform directive force on all courses, so that if any deviations remain they will be small in amount and regular in their changes.

ART. 128. Heeling Error.-So far it has been assumed that the ship is upright, but when she has a list from the pressure of the wind, or shifting of cargo, or unequal trimming of bunker coals, further deviations-sometimes very large, and known as heeling error occur on northerly and southerly courses, but disappear on east and west courses.

The Cause of Heeling Error.—(a) It has been already explained that there is a vertical component of sub-permanent magnetism as well as a horizontal. The upper pole of this force, nearest to the steering compass aft, is blue if the ship is built head north (Fig. 209), and red if built head south (Fig. 210), and so long as the ship is upright, it only tends to attract the N. point of the needle downwards or repel it upwards, according as the upper pole is blue or red, without causing any deviation. The case is different when the ship heels, for then this upper pole is no longer in the vertical plane passing through the centre of the compass, but

towards the higher side, deviating the N. point of the compass towards the higher or lower side. This effect is evidently

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greatest on N. and S. courses, and nil on E. and W. courses, because in the former case the force acts at right angles to the needle, and in the latter case in the plane of the magnetic meridian.

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(b) The upper pole of vertical iron below the compass affects the compass in exactly the same way as the sub-permanent pole when the ship heels, only it must be remembered that vertical iron has the blue pole uppermost only in N. magnetic latitude, and red pole uppermost in S. magnetic latitude, being neutral on the magnetic equator. Therefore the heeling error from this cause decreases on sailing towards the magnetic equator, and vice versa.

(c) The athwartship iron, which gives coefficient D when the ship is upright, acquires a vertical component when she heels, which is of the same nature as the magnetism of vertical iron, and is subject to the same changes-the higher end being blue in N. magnetic latitude, and red in S. magnetic latitude.

It is evident that in some cases these three causes of heeling error would act together, and the heeling error would be large. In other cases the first would be opposed to the other two, and they would tend to neutralize each other, and give a small heeling error.

Their combined forces might be represented by a single force

acting vertically under the centre of the compass, which would therefore be the resultant of

(1) The vertical component of sub-permanent magnetism. (2) The induced magnetism of vertical soft iron.

(3) The vertical component of the induced magnetism of transverse beams.

This resultant force is called if the balance on the side of the greater is blue uppermost, and if red. The total effect may be summed up by saying that a + vertical force causes a heeling error towards the higher side, and a force a heeling error towards the lower side (Figs. 213 and 214).

FIG. 213.

FIG. 214.

Experience shows that in the majority of sailing ships the force below the compass is + in N. latitude and in S. latitude, with the result that N. of the magnetic equator the ship is drawn to windward on northerly courses, and to leeward on southerly courses. South of the magnetic equator the rule would generally be reversed. The same rules apply to steam ships, if "higher side" is substituted for "windward," etc.

Navigators make a practical application of these facts by keeping" away" on northerly courses, and "closer" on southerly courses, when north of the magnetic equator, and the reverse when south of it.

Since the vertical force draws the N. point of the compass to one side or the other when the ship heels, it is called a coefficient C, although it would be more logical to distinguish it in some way from the coefficient C already discussed, due to horizontal force, say by using C1 to represent it.1

Like coefficient C of horizontal sub-permanent force, the new coefficient C1 causes no deviation (heeling error) when the ship's head is E. or W. (magnetic), and the greatest deviation on N. and S. courses, because in the former case it acts in the plane of the magnetic meridian, and in the latter at right angles to the needle.

ART. 129. Compensation.-The heeling error is usually

For a similar reason, the part of coefficient B due to vertical iron before or abaft the compass might be distinguished as B1.

compensated by means of one or more magnets placed vertically under the compass, and capable of being moved up or down. There are two modes of procedure:

(1) The ship's head being at N. or S. magnetic, she is caused to heel, say about 10°, and the error observed. Then the magnet is placed in position, and moved up or down, until the compass points correct magnetic N.

(2) The card having been removed, and the ship upright, with her head E. or W. magnetic, a dipping needle, weighted so as to lie horizontal when in the plane of the magnetic meridian on shore, is placed on the pillar in the centre of the bowl, and lying N. and S. Should the red end dip, showing a blue force below the compass, the red pole of the vertical magnet should be uppermost, and vice versa, being moved up or down until the needle is again horizontal.

In either of these methods, what has been done is to counteract the force below the compass by a force of opposite name; by observation of the heeling error in the first case, and in the second case by observing the vertical action of the force below the compass on the dipping needle.

In a compensated compass the vertical force of the athwartship iron (producing coefficient D) is counterbalanced for moderate angles of heeling by a vertical force developed in the correctors; which leaves only the effects of vertical iron and the sub-permanent magnetism to be dealt with.

The Flinders bar increases the heeling error, for its upper pole is of the same name as that of the vertical iron it is intended to counteract when the ship is upright. Therefore the compensating magnet has to neutralize the combined vertical forces of sub-permanent magnetism, vertical iron, and Flinders. bar. But as the two latter change with change of latitude, it is evident that this mode of compensation is effective only so long as the ship remains in the same latitude, and constant shifting of the magnet would be necessary as the ship proceeds north or south, and probably a reversal of the poles on the other side of the magnetic equator.

The only efficient compensation would be to use the magnet for the part due to sub-permanent magnetism only, and a vertical soft iron bar, having its lower end above the compass, to counteract the vertical force of soft iron below the compass. Some arrangement might possibly be made for the purpose in the construction of the binnacle.

It will now be understood that the principle of compensation is to counteract the existing magnetic forces in a ship by employing counter equal forces of the same kind, to act on the compass needles.

The order of procedure recommended in the "Elementary

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