DEVIATION OF THE COMPASS.

259. One of the most practical, as also one of the most useful, purposes to which the laws explained in the preceding chapters of this work can be applied, is to find the effects of the iron used in the construction of ships on the compasses placed in them; and the reader who thoroughly understands these laws will have no difficulty in deducing the results which follow.

260. The iron of which a ship is constructed is neither wholly hard nor wholly soft, but of an intermediate character; and the more nearly it approximates to hard the more capable it is of the retention of magnetism when it has been subjected to any process of magnetisation. In an iron ship intense magnetism is developed by the bending, twisting, and hammering to which the iron is subjected; N. (red) magnetism being developed in the part of a ship which is below and towards the north, and S. (blue) magnetism in the part which is above and towards the south; and so far as the hull is concerned it is of that kind which is now called sub-permanent, because it has not the precise consistency of a steel bar magnet.

261. In the northern hemisphere all vertical or upright bars, such as stanchions and angle-irons composing the frames of ships, are magnetised by induction, the lower ends being north (red) poles, the upper ends south (blue) poles, the upper end attracting the north pole of the needle held near them. On the other hand, in the southern hemisphere, these conditions are reversed; the upper ends of vertical iron are north poles, repelling the north pole of a compass needle and attracting the south pole. On the magnetic equator, where there is no dip, vertical soft iron has no polarity, because its position is at right-angles to the earth's line of force or dip. It is different with horizontal pieces of soft iron; they exert the same influence on a compass needle in both hemispheres, and in all latitudes.

The hull of an iron ship acts as a permanent magnet on compasses placed outside the vessel as well as those placed inside; an iron ship must therefore be viewed in its effect on a properly placed magnet rather as one great magnet, than as an aggregation of smaller magnets.

Keeping in view that the inductive effect from the earth's magnetism is greatest in the line of the dip, and the existence of a neutral equatorial plane at right-angles to the line of dip in spherical bodies, we are prepared to see that each iron ship must have a distinct distribution of magnetism depending on the place of building, and the direction of the head and keel while building; the ship's polar axis and equatorial plane conforming more or less to the line of dip of the earth at the place where built, and a plane at right-angles to that line; abundant observation and experiment have proved this important general principle.

262. To illustrate this principle: let us suppose, as in the following Figs. 77, 78, 79, and 80, that four iron ships, or four composite-built ships, with ribs, beams, stanchions, and deck girders of iron, are building on the cardinal points of the compass, in a port in England where the line of the earth's total magnetic force is inclined 70° to the earth's horizontal magnetic force, or in other words, where the dip of the needle is 70°.

Fig. 77 shows the magnetic state of a ship built head North magnetic. The line marked Dip passes through the centre of the ship; it shows the direction of the line of the earth's magnetic force. The line marked Equatorial or Neutral line is the line of no deviation, and runs at right-angles to the Dip. The after body of the ship, or the portion which is shaded, has

8. (blue) polarity, and the fore body, or white portion of the figure, N. (red) polarity; the upper part of the stern would have the S. (blue) polarity developed in a high degree; the lower part of the bows would have the N. (red) polarity equally developed. At the stern the north end of a compass needle would be strongly attracted; at the bow the south end of the needle would be strongly attracted; while a compass placed outside of the ship's topsides, above the line of no deviation, the north end of the needle will be attracted; if it be placed below that line the north end of the needle will be repelled and the south end attracted, in accordance with the law of magnetism. Fig. 78 represents the magnetic condition of a ship built head south. It will be seen by comparing Fig. 78 with Fig. 77 that the conditions are reversed; in Fig. 77 the magnetism of the after body of the ship is south (blue), while in Fig. 78 the after part of the ship possesses north (red) polarity; now the fore body of the ship has S. (blue) polarity, while in Fig. 77 it has N. (red) polarity; the upper part of the bow has S. (blue) polarity developed in a high degree, and the lower part of the stern N. (red) polarity equally developed. The N. (red) polarity of the stern repels the north end of the compass needle, and the S. (blue) polarity of the bow attracts it. The dotted line crossing the equatorial line in Figs. 77 and 78 shows the probable position of the neutral line after the ship has been sometime afloat, with her head in an opposite direction to that in which she was built, or after she had made a voyage.

The place of little or no deviation in a ship built head north is towards the bow, but in a ship built head south, towards the stern.

Fig. 79 is intended to show the magnetic state of a ship built head East. The whole of the upper part of the ship has S. (blue) polarity; the whole of the lower part has N. (red) polarity; but the S. (blue) polarity predominates on the starboard side, and the north end of a compass needle, if carried at the usual height of a compass along the amidship line of the upper deck from end to end, is attracted to the starboard side.

In Fig. 80, ship built head West, the magnetic conditions of Fig. 79, head East, are reversed; the whole of the upper part of the ship has still S. (blue) polarity, and the lower N. (red) polarity; but the magnetism of the port side of the upper works is developed in a higher degree than the starboard side, and the N. end of a compass needle, if carried along the upper deck from end to end, would be attracted to the port side. In other words, in these

ships the whole of their decks have a S. (blue) polarity, yet in that part which was North while the ship was being built, this S. (blue) polarity is developed in a less degree than on the opposite side, consequently the N. point of the compass is drawn towards that part of the ship in which the S. (blue) polarity is developed in the highest degree.

Theoretically, there should be no spot of no deviation on the deck of ships built East or West.*

This disposition of magnetism, and its results as here described, appertain to the northern hemisphere.

The diminution of the directive force is greater if the ship has been built East and West than if built North and South; also the deviation is less symmetrical, regular, and permanent than those of a ship built North and South.

The equatorial or neutral line rising to the deck forward in ships built head North, and aft in ships built head South, might lead to the supposition that a compass placed in one or the other position would not be effected by the magnetism of the ship; it is well to place a compass aft in ships built head South, and as far forward as possible head North. But the line or

From the special magnetic properties developed in a ship according to her position when building, it follows that a compass aft, in the usual place of the steering binnacle, the character of the deviation-though not the amount-may be approximately represented in a tabular form, as follows:Approximate magnetic direction of ship's head while building.

N.
N.E.

E.

Approximate easterly deviation occurs

Maximum westerly deviation
when ship's head by compass is near when ship's head by compass is near
W.
N.W.
N.

E.

S.E.

S.

space along which the magnetism of the ship has no influence on the compass is not easily discovered, and if it were ascertained for any given latitude it would be found to have changed on reaching another magnetic latitude.*

Fig. 81 represents an iron ship built head North in Australia, with a dip of about 68° South. In this ship the shaded part showing S. polarity lies below the equatorial line. It will be useful to compare this figure with Fig. 77, and mark the difference in the magnetic state of the two ships.

A little attention to the above diagrams will give the seaman a rough idea of the distribution of magnetism in iron ships; but it must be borne in mind that all large detached pieces of iron in a ship, such as iron masts, funnels, cylinders, and other masses of vertical iron, are independent magnets; in north magnetic latitude, their lower ends being north poles, their upper ends south poles.

The compasses of composite ships with iron frames and iron deck beams, are affected in the same way as those of ships built wholly of iron.

263. The errors of the compass may be classed under the heads Variation, Deviation, or Local Attraction.

264. Variation is the angle included between a magnetic and terrestrial meridian. This error arises from the magnetic poles not being coincident with geographical ones, and is due entirely to the influence of the earth on magnetic needles (see pages 75 and 96).

* There is a very common error prevalent respecting the best position on the ship's deck for a compass. Some of the earliest writers lay it down as an axiom that on the deck of all iron ships there is a neutral point which should be sought out for the fixing of a compass. That an apparently neutral point does frequently exist is admitted. But this very point is most frequently a very dangerous position for fixing a compass. It is neutral only while the ship is in one locality. For example, in the S.S. "Fidelia," in Table Bay, just before the saloon skylight, there was no deviation. "There existed sub-permanent magnetism B=+34°, and induced magnetism in vertical iron B =— 34°, and the resultant of these two errors was therefore zero. But had she returned to Sunderland the B=34° would have been changed to B = + 51°, which, when compounded with the results of subpermanent magnetism, would have produced a horizontal force exceeding that of the earth's."