Fig. 4 represents the magnetic condition of a ship built head south. It will be seen by comparing Fig. 4 with Fig. 3 that the conditions are reversed; in Fig. 3 the magnetism of the after body of the ship is south (blue), while in Fig. 4 the after part of the ship possesses north (red) polarity; now the fore body of the ship has S. (blue) polarity, while in Fig. 3 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) polartiy of the bow attracts it. The dotted line crossing the equa torial line in Figs. 3 and 4 shows the probable position of the neutral line after the ship has been some time 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. 5 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. 6, ship built head West, the magnetic conditions of Fig. 5, 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.

The deviation in both cases is rarely large, but less regular than in ships. built head South.

T

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

Fig. 7 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. 3, and mark the difference in the magnetic state of the two ships.

220. 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.

221.

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.

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 :

222.

DEVIATION OF THE COMPASS.

The Deviation of the Compass is the angle through which the magnetic needle is deflected from its natural position by the disturbing force of iron near it, that is, the angle included between the magnetic meridian and a plane passing through the poles of a compass needle.

The deviation is named East or West according as the north point of the compass so disturbed is to the east or west of its natural position.

Deviation consists of two principal parts, the Semicircular and the Quadrantal, following different laws, and requiring two different kinds of compensation; there is sometimes a third part of small amount called the Constant.

223. In the case of iron ships, as in that of iron bars (215), percussion and vibration, by hammering in rivetting, render the iron of which the vessel is constructed more susceptible to the inductive force of the earth, and causes the magnetism, which the iron of the ship thus acquires, to partake more of the character of permanent magnetism. Still this subpermanent magnetism undergoes a considerable diminution by being submitted to percussion, with the ship's head in a different position to that in which it was when she was being built, and especially if in a contrary direction. But the iron of which a ship is constructed always retains a large amount of this subpermanent magnetism as long as it remains in the form of a ship. The deviation arising from subpermanent magnetism is greater than that which is the result of transient induced magnetism. The polarity of the ship's magnetism, while she remains on the stocks, takes the direction of the earth's line of force or dip, and its effects on compasses will evidently depend on the direction the ship's head was whilst being built. Taking the case of a ship built head north (Fig. 3, page 135), the fore part of the ship has acquired north magnetism, and its action will be precisely the same as that of the north pole of a magnet; hence, on northerly courses, the north end of the compass needle will be repelled, and the directive power of the needle will be diminished. On southerly courses the north end of the needle points towards the stern, which has acquired subpermanent south magnetism, then the directive power of the needle is increased. On easterly and westerly courses the effects on the compass are greatest, since the force acts at right-angles to the needle; and on all intermediate positions of the ship's head the disturbances due to such positions are intermediate. As the ship's head is brought east of north, repulsion of the north end of the needle takes place, and westerly deviation is the result, and it reaches its maximum value when the fore-and-aft line of the ship is at right-angles to the needle; beyond that position the fore part of the ship attracts the south end of the needle, and westerly deviation is still the result. This attraction continues until the ship's head reaches south, when the line of action of the ship lies in the same direction as the needle, and no disturbance occurs, but the directive power of the needle is greater. On bringing the ship's head round west of south, the south pole of the needle still continues to be attracted, which causes easterly deviation, and it again attains its maximum when the fore

and-aft line of the ship is at right-angles to the disturbed needle; this must occur to the north of west. After that point has been reached by the ship's head, the fore part of the ship repels the north end of the needle, easterly deviation still being the result until the ship's head is again at north. Thus we find that in an iron ship the disturbance of the compass is little or nothing when her head is on or near the points to which her head or stern were directed while building, and is greatest when the ship's head is directed to the points of the compass that were abeam while on the building slip; and, moreover, that easterly deviation is caused when the ship's head is in one half of the compass, and westerly deviation in the other. The deviation caused by subpermanent magnetism, and the effects of magnetism induced in vertical iron, has received the name of Semicircular Deviation, from producing opposite effects when the ship's head is on opposite semicircles of the compass, as the ship's head moves round a complete circle of azimuth. This error is caused by the subpermanent magnetism acquired in building, and the magnetism induced in vertical iron. The part due to subpermanent magnetism remains the same in kind, though different in amount, in all latitudes, unless the ship be subjected to strains or other mechanical violence. The part caused by the magnetism induced in vertical iron changes with a change of geographical position, or more correctly, as the dip changes, and is of contrary names on opposite sides of the magnetic equator, that is, if westerly deviation be produced on one side, eastorly will be produced on the other. At the magnetic equator the earth's magnetism acts horizontally, and vertical soft iron will have no magnetism, and the semicircular deviation arising therefrom will disappear.

As a general rule the magnetism producing semicircular deviation, in a ship built in north magnetic latitude, attracts the north end of a compass needle to that part of a ship which was south from the compass while building; hence, the semicircular deviation in iron ships is generally represented by the effect of a magnet at the part of the ship which was south in building, with its south end towards the compass. Thus, in a ship built head north, the north end of the needle is drawn towards the stern. The following table will show the part of a ship towards which the north end of a needle is generally drawn, that is, the position of the permanent south pole developed in the process of construction.