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ON COMPASSES, AND THEIR ADJUSTMENT IN

IRON SHIPS.
(Concluded from page 903.)

HE HEELING ERROR. All that has been said so

far relates to a ship when upright, that is, to a ship on an even beam and keel, which should be her

position when swung for the deviation of the compass, or for the adjustment of any of them; and be it remembered that though the compasses have been adjusted in respect to the semicircular and quadrantal co-efficients, the effect of the correction is not such as to efface, though it may mitigate, the heeling deviation.

The following diagrams, 32 and 33, in respect to a ship built head towards North in the northern hemisphere, illustrate the effect of the magnetism as she heels over; and the worst possible position for a compass in such a ship is near the stern, for there all the forces conspire to magnify the error.

[graphic][graphic][subsumed][subsumed][merged small][merged small]

It is easily seen that, as the ship heels over, the transverse iron, such as the deck beams, becomes magnetic as it inclines, and the apper or weather end, s, (with blue magnetism) attracts the N. (red) point of the needle; further, the upper end of the soft iron, which, before heeling, acted vertically below the compass and did not disturb the horizontal needle, is now with its s, or blue magnetism, brought out to windward, and is consequently an additional force pulling the N. point of the needle to windward; to this must be added the effect of the vertical force of the sub

manent magnetism, which, according to circumstances, may act

upwards or downwards. The heeling deviation is, in this case, to the high or weather side, whether heeling to starboard or port.

Since, however, the effect of induction will be changed at the magnetic equator, and will be reversed, as the upper end of the iron acquires magnetism south of the equator, the heeling error, to state a general case, arises from the following causes :

1. Vertical induction in transverse iron, which draws the N. end of the needle to windward in N. latitudes, and to leeward in S. latitudes.

2. Vertical force arising from sub-permanent magnetism; and vertical induction in vertical iron; which, in the usual position of the steering (aft) compass, draws the N. end of the needle to windward in ships built head North, to leeward in ships built head South.

Hence, an iron ship, built head North, will generally have a large heeling error to windward in N. latitude; and a small heeling error, which may be to windward or to leeward, in S. latitude.

Also, an iron ship, built head South, may be expected to have a small heeling error to windward or to leeward in N. latitude, and a considerable heeling error to leeward in S. latitude.

The heeling error being a maximum on the North and South points by compass, and nil at East and West, it is evident that the coefficient C is that which must be most affected, and if the deviation for an upright ship be,

d B sin % + C cos z + D sin 2 z we shall have, for a ship heeling n degrees, with c taken to be the change in C for one degree of heel, the following formula

dn = B sin % + C + c no) cos z + D sin 2 %

d + c no cos z that is, the heeling error alters proportionally to the number of degrees of heel and the cosine of the azimuth of the ship's head.

It is possible to put this, for practical purposes, in a more simple form. Since, in the northern hemisphere, in the majority of iron ships, the North end of the compass needle is drawn to windward (to the weather, or high, side) when the binnacles are above

or, dn

the upper deck, the following results, as neatly put by Captain Evans, R.N., would arise from a disregard of this heeling error:

“If the ship be kept steady on one compass course, she will be found to windward of her supposed position when on northerly courses ; and to leeward on southerly courses. If she be steered steadily for a fixed point on the horizon, she will appear to fall off as she heels on northerly courses, and to come up on southerly courses.' Therefore, as a general rule, it should be borne in mind " that in steering by compass and wishing to make a straight course, we must keep away, by compass, on either tack, as the ship heels, when on northerly courses ; and keep closer to the wind, by compass, on either tack, when on southerly courses."

Every seaman will apprehend this plain language ; but to express the heeling deviation in terms of the deviation when upright, the following are the results : ON NORTHERLY COURSES, —

Starboard tack, E. dev. is increased, W. dev. is decreased.

Port tack,- W. dev. is increased, E. dev. is decreased. ON SOUTHERLY COURSES, —

Starboard tack,-W. dev. is increased, E. dev. is decreased.

Port tack,-E. dev. is increased, W. dev, is decreased. And it can easily be understood that when the upright deviation

is small in amount and decreases from heeling, it may go

to the extent of being reversed in name. Observe further that there is a correspondence between the star

board tack on Northerly courses and the port tack on Southerly courses; also between the port tack on Northerly

courses and the starboard tack on Southerly courses. When the North end of the compass needle is drawn to leevard, the rule above is of course reversed.

As some may better understand the naming of the heeling error if given according to the tack, it can be expressed as follows: 1. When N. end of the compass needle is drawn to the weather

sideOn starboard tack, heeling error is E. on Northerly courses;

W. on Southerly courses.

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On port tack, heeling error is W. on Northerly courses ;

E. on Southerly courses. 2. When N. end of the compass needle is drawn to the lee sideOn starboard tack, heeling error is W. on Northerly courses ;

E. on Southerly courses. On port tack, heeling error is E. on Northerly courses ;

W. on Southerly courses. Finally it may be observed that1. If, on changing tack, the course is changed from Northerly

to Southerly, or from Southerly to Northerly, the heeling

error does not change its name. 2. If, on changing tack, the course still remains Northerly, or

still remains Southerly, the heeling error changes its name;

also 3. If, without tacking, by a change of wind the course changes

from Northerly to Southerly, or from Southerly to

Northerly, the healing error changes its name. These observations do not apply to the amount of heeling error, which of course, depends on the extent, or number of degrees, of heeling.

The amount of heeling error with the ship's head North or South, on which points it is a maximum, varies greatly in different ships. Something between foto 2° for every degree of heel is by no means unusual, and it has been found to be as large as 3o; in very rare instances the latter amount may be exceeded, but there is no sufficient proof of this. The amount of heeling error in one ship is no criterion for estimating that in another, though both ships may have been built in the same magnetic direction.

To ADJUST THE COMPASS, OR CORRECT THE DEVIATION, WITH PERMANENT MAGNETS AND Soft IRON.—It is taken for granted that the reader understands the use of azimuthal bearings—it may be of a distant object on shore-or reciprocal, as is sometimes compulsory-or of a celestial body, as the sun, a planet, or a fixed star; any of which may, according to circumstances, and the locality, be employed while swinging a ship, and bringing her head to any given number of correct magnetic directions, for the purpose

of making a deviation table for the various points of one or more compasses, or of correcting the deviation by permanent magnets and soft iron.

But it should be well understood that, for use on board ship, no instrument is preferable to the dumb card, or compass card without a needle, from the facilities it offers in the comparison of compass readings, and the rapidity with which the correct magnetic direction of the ship's head may be determined.

In placing a compass have it well raised above the iron beams, as far as convenient from an iron bulkhead, and, if possible, not within six or seven feet of any vertical iron. Take care to have the lubber's point in the midship fore-and-aft line.

The card of a compass, to be properly adjusted, should not have less than two parallel needles of great directive force, placed edgeways, fixed to the card, and 60° apart. The pivot on which it moves should be of hard steel, well pointed but not needle-like, and perfectly smooth. The central stone should be a hard gem, with the concavity well rounded and polished; an agate is not to be trusted, though some are very hard. The card resting on its pivot should be truly horizontal. The ship should be perfectly upright.

These conditions being fulfilled we may proceed to adjust compasses ; and if you choose, and cannot trust your eye, you may draw two chalk lines-one fore-and-aft, and the other athwartship -crossing each other under the centre of the compass.

With the ship's head on correct magnetic North or South, if the compass by its indication shows any deviation, the compensating bar-magnet must be placed athwartship, either before or abaft the compass (it matters not which), the middle of the magnet on the midship fore-and-aft line with the centre of the compass, and the N-marked end (red pole) of the magnet, directed to that side of the ship towards which the N end of the compass-needle appears to be drawn: thus, the red (N) pole of the bar magnet must be to starboard if the deviation is Easterly with the ship's head North, or Westerly with the ship's head South (this is + C); but the red (N) pole of the magnet goes to port if the deviation is Westerly with the ship's head orth, or Easterly with the ship's head South (this is - C). Begin

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