ON THE MARINERS' COMPASS.-By Mr. W. Walker, Master R.N.

(Continued from p. 655.)

EVERY body who has been much at sea, or, who has been in the habit of watching the motion of the Mariners' Compass, must have observed that the compass card does not remain very steady in its bowl, during bad weather. When the ship lurches heavily, or rolls from side to side, the compass card oscillates several points from the actual direction of the ship's keel: when a ship is running before the wind, in a high sea, and rolling, perhaps 15 or 20 degrees, on each side of the perpendicular, her compass card may swim or vibrate a couple of points on each side of the course!

To remedy this oscillation of the compass card weight is added in the shape of wax, brass bars, &c., for it has been considered that this vibration arose from mechanical action. There is, however, no "law of mechanics" whereby this action can be satisfactorily explained!

We have already explained how the poles of a piece of iron are to be found by means of the magnetic dip. Now the magnetic dip has reference to the earth, and not to a ship and the iron she may contain. We may for our present purpose, regard the earth as a fixture, but a ship when afloat and at sea, is a moveable body, changing her position and direction; inclining by the force of the wind on her sails, or rolling and pitching about by the action of the waves on her hull! Now the magnetic dip of the needle, and the consequent magnetic polarity of the iron that a vessel may contain, is always referable to a plumb line; (because we measure the dip from a vertical) consequently the ship and her contents are constantly changing their relative positions to the dip, and also to the magnetic attractions and repulsions which every article of iron that the vessel may contain receives by induction from the earth. Whenever a ship changes her position, or her inclination, a new magnetic force is brought to bear upon the compass; and when the ship rolls alternately from side to side, equal and opposite magnetic forces act upon the compass needle, and cause it to oscillate on each side of the true magnetic direction of the ship's keel. I beg to call the mariner's special attention to this part of our subject, and to refer him to Fig. 6, in our last experiment, where it is shewn, that an iron bolt laid in an east and west direction by the side of a compass (as W, E,) will not affect a compass needle, even if it be within an inch or two of Fig. 7.

the compass. Now, let us suppose that there is a loug iron gun on each side of a ship's quarter-deck, and exactly abreast of the binnacle. When the ship's head is either north or south, the guns will be in an east and west direction, and like W, E, (in Fig. 6,) will not derange the compass needle so long as the ship remains quite upright. But let the

vessel be inclined as in Fig. 7, the magnetic polarity of the two

guns, and also of every bolt, bar, or nail that may be fastened through the sides of the vessel, will have changed its place in the iron. Let the direction of the ship's head be north, and her inclination be to starboard, then the breech of the lee gun, B, will attract the north point of the needle, and its muzzle will attract, the south point.

On the other side of the ship, the breech of the weather gun, A, will attract the south point of the needle and repel the north point of the compass, so that the north point of the compass card will be drawn to leeward by the gun, B, and driven to leeward by the gun, A, whilst the south point of the needle is drawn to windward by the gun A, and driven to windward by the gun B. If the water be smooth, and the ship's inclination be permanent, this kind of local attraction will permanently derange the ship's course; but if she roll from side to side, the compass card will also vibrate on each side of the course.

If the ship's head be north (as before) but her inclination to port, by a strong easterly wind, the polarity of the guns will be inverted: that Fig. 8.

is to say, the lee gun A, (Fig 8,) will attract the north point of the compass needle, and draw it towards the lee side of the ship; and the weather gun B, will repel the north point and attract the south end of the compass needle. In this way the compass, instead of indicating a course at north, may shew a N. E. or a N.b.E. course, in smooth water and in north magnetic dip, but should the vessel be running before the wind, and rolling heavily from side to side, so that at every roll of the vessel, the inductive polarity of the iron within her is actually transferred from one side to the other, the compass card must necessarily go on in an endless oscillation, unless means be devised to prevent it. The ordinary means resorted to by seamen, is to increase the weight of the card, that is to say, to use a more sluggish instrument.

It was owing to the vibratory motion of compass cards mounted in wooden bowls, that brass bowls were substituted for the wooden one. The fact is, that copper, or brass, is capable of receiving an inductive magnetism from a magnetic needle near it; and although a brass bowled compass may not vibrate like a wooden bowled compass, it is beyond a doubt, that a card mounted in a brass bowl, is more sluggish in fine weather. It is even frequently necessary to have small lines attached to what are called heavy compasses, in order that the helmsman or quarter-master may agitate the compasss and cause it to traverse in light winds and smooth water. The means, therefore, that have hitherto been adopted by seamen, as well as by the makers of ship's compasses, have not been founded on sound principles. These oscillations were supposed to arise from some principle in mechanics not easily understood. Whereas the vibration of the needle arises from a change of place in the magnetic poles of the iron, and other things that enter into the construction of the vessel and her contents; and it will be shewn hereafter how these vibrations, and in fact the local attraction generally may be rectified and got rid of.

But in order to convince seamen that the oscillation of their compass arises from magnetic action, the following experiments may be made in any vessel. Place a short plank (in equilibrium) upon any thing, so that it may rock, or roll from side to side like the rolling of a ship. Place a compass upon its middle, the plank being in an east and west direction; and it will be found that the compass card will not vibrate, although the plank be moved or heeled from side to side. Place now a large bolt or bar of iron on each side of the compass, and it will be seen that the compass card will swing or vibrate, if motion be given to the plank.

The experiments we have been explaining prove beyond any doubt, that iron is magnetic, that it has magnetic poles, and that these poles are always referable to the direction of the dip of the magnetic needle, and do not remain in a permanent position in the iron. Any attempt that may be made to correct the local attraction of a ship's compass, or the oscillation of the needle in stormy weather must necessarily fail, unless the operator understands clearly the philosophy of his subject. Professor Barlow failed, because he believed, that the central action of all the iron in a ship remained constant, in all parts of the world, and he did not believe that iron was polarised as we have shewn it to be.

"I am the more anxious to establish this point," says the author, "in consequence of its immediate connection with the method I have proposed for correcting the errors of a ship's compass, which has been objected to, on the ground, that according to the theory we have been controverting, the central action of all the iron on board would not remain constant under all dips, and in all parts of the world, but if the hypothesis I have advanced be correct, then the central action of any irregular mass of iron, will be in the centre of attraction of its surface, whatever may be the magnetic direction, and must necessarily remain the same, while the iron and the point from which its action is estimated, preserve the same relative situation; as in the case with the iron of a vessel and its compass."+ Now although the iron in a ship and her steering compass do actually preserve their relative position within a ship, yet the magnetic energy of the iron and its inductive polarity, do not preserve their relative position in the ship, nor to the binnacle; for we have proved that the magnetic poles of a piece of iron in a ship is referable to the earth and not to the ship.

The Astronomer-Royal published in the United Service Journal for June, 1840, practical rules or directions, " for correcting the compasses of iron-built vessels." Now vessels built entirely of malleable iron, will hold inductive magnetism, and many of its pillars will, in this country, exhibit a permanent magnetism. For independent of that magnetism which an iron vessel may receive, in the progress of building (and which we have already noticed) the upright bars may conduct upwards (See Fig. 6,) a magnetism from below or from the boilers, &c.

The practical directions published by Mr. Airy, although they may be of great practical utility in any iron built vessel that may navigate the English Channel, or in fact, around the British Isles; yet the plan he has proposed and the directions he has given are not applicable for

* Barlow on Magnetic Attraction, 2nd Edition, 1842, p. 307. + Ibid. p. 182.

distant regions. His plan is to find the local magnetism of the vessel upon its compass, by swinging the ship in the usual way, and then to correct the local attraction of the ship and her contents by means of permanent magnets, placed at a convenient distance from the compass. His method therefore is, to correct the inductive magnetism of the malleable or cast iron fabric of the vessel, by means of permanently magnetic steel bars: that is, to correct, or cancel in one hemisphere, by a constant quantity a magnetic agency that may vanish, or from being positive will become negative in the other hemisphere.

In iron-built vessels the compass must always be liable to great derangement and irregularity and less dependence should be put upon their dead reckoning. In ships built generally of timber, the local attraction upon the compass presents nearly a uniform character although the amount of deviation may vary in different ships and with different cargoes. The rudder is fixed at the stern, the steering wheel is near the rudder, and upon the upper deck, consequently, the compass must be placed near the helmsman, that is upon the upper deck and near the after end of the vessel. The principal quantity of iron in a ship will therefore, be before and below the compass and the nearest inductive magnetic poles in the iron will act more powerfully on the compass than the more distant and opposite poles. The result is that in our hemisphere the north point is drawn forward, and in south magnetic latitude it is the south point of the compass that is drawn forward by the ship's local attraction, and the greatest effect takes place when the ship's head is nearly east or west.

The amount of attraction or repulsion of iron upon a ship's compass, will depend upon the quantity, mass, or magnitude of the metal, and its distance from the compass needle. The disturbing magnetic action of the iron increases as its distance from the compass diminishes, in the inverse duplicate ratio of the distance; that is to say, if we place an iron bar at four feet from the compass needle, its magnetic action will only amount to one-fourth of what it would be at two feet, and onesixteenth of what its force would be at one foot distant, and so on, the force increasing in the inverse duplicate proportion of the distance. We see then, that a very small quantity of iron, as an iron bolt in the corner of a hatchway, or skylight, if near the binnacle, may act upon the compass as powerfully as a gun would act when secured in a port at the side of the ship.

The derangement of a compass by the magnetic action of masses of iron, may be ascertained at sea, by its oscillation, and by its indicating different bearings of a distant object, when the ship's head is in different directions, at the same anchorage. Another sign of the existence of local attraction in a ship at sea is noticed when beating to windward, say with a northerly wind, when the ship appears, by the compass, to lie within four or five points of the wind. Whereas, when beating to the southward she may appear to be no closer to the wind than six or seven points. These anomalous appearances in the direction of the ship's head, arise from the north point of the compass card being drawn forward on both tacks by the local magnetism of the vessel. Whenever these symptoms appear a compass should be placed on the forecastle of the ship, and the magnetic direction of the ship's head on both com

passes noted. The one in the binnacle will have its north point drawn forward, and the compass forward will have its north point drawn aft. Hence the correct magnetic bearing, or direction of the ship's head will be intermediate. When doubts exist in a merchant ship about the correctness of the course, the above plan of carrying a compass forward and comparing it with the one abaft affords an excellent check against any local attraction that may arise from receiving a new cargo, or from making changes in the stowage in a vessel.

The magnetism which the iron within a ship receives from the earth in all latitudes, will act upon the steering compass in the following

manner:

1st. In north magnetic dip, the higher or upper parts of the iron being north poles, the north point of the compass card (which is a south magnetic pole,) will be drawn forward in the vessel, and the south point will be repelled towards the stern, and hence the compass will indicate a course farther to the northward than the ship steers; consequently, the ship will be to the southward of her reckoning.

2nd. In south magnetic dip, the highest or upper parts of the iron will possess south magnetic poles, and the south point of the compass needle, (which is a north magnetic pole,) will be drawn towards the ship's head, and the north point repelled towards the stern, and hence the compass will indicate a course farther to the southward than the ship steers, and she will be found to the northward of her reckoning.

3rd. In north magnetic dip, and by reason of the changeable polarity of the iron in a vessel (Fig. 7,) as for example in a man-of-war, the north point of the compass card is drawn towards the lee side, and the south point is attracted towards the weather side, whenever the ship is inclined by the force of the wind on her sails; or, in fact by any other means, as by shifting her cargo.

4th. In south magnetic dip, and when a ship is inclined from an upright position, the south point of the compass is drawn to leeward, and the north point is drawn to windward by the induced magnetic poles of iron being transferred from end to end of a gun or bolt in a ship's side, &c. But when a ship rolls from side to side, in regular succession, the compass card obeys the magnetic impulses of the changeable polarity in surrounding objects, and goes on in regular oscillations.

These are generally the conditions of the local magnetism of all sailing vessels, and of almost all wooden-built steam vessels, whose compasses have not been corrected by artificial means, or removed beyond the sphere of the ship's local magnetism. If the local attraction of a ship has been correctly ascertained by swinging her round, and the amount of local magnetism noted upon each of the two-and-thirty points of the compass, then these corrections may be safely applied to the courses, so long as the ship continues to be navigated in the same amount of magnetic dip; but, if the ship's local attraction has not been correctly found, we may deduce the following practical results from the principles we have been propounding.

1st. Almost all ships will be found nearer to the magnetic equator than the dead reckoning will place them: thus, in England, or in north magnetic latitude, ships get to the southward of their reckoning; but