265. Deviation is the angle by which the needle is deflected from the magnetic north by the action of the iron of a ship; 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.

266. Local Attraction is the error caused by some disturbing force unconnected with the ship itself, or cargo, but belonging entirely to the locality, such as iron cranes, mooring posts or chains, an iron vessel near, &c.; or volcanic or other magnetic bodies like those found in Elba, St. Helena, &c.

Deviation is distinguishable from the other two errors by its depending, both for amount and direction, on the position of the ship's head and the angle at which the ship is heeled; whilst the other two are independent of the action of the ship.

267. How Compasses act in Iron Ships.-In the case of iron ships, as in that of iron bars (page 322), 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 magnetism, after launching, 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 magnetism as long as it remains in the form of a ship, and for this reason, as, also, to distinguish it from the permanent magnetism of steel magnets, it has received the name of sub-permanent magnetism. The deviation arising from sub-permanent 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. 77, page 337), the fore part of the ship has acquired north (red) 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. southerly courses the north end of the needle points towards the stern, which has acquired sub-permanent 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

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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 rightangles 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 as her head is moved in azimuth round an entire circle 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 that between these two points are two semicircles, in one of which the deviation is westerly, and in the other easterly, and is greatest, or maximum, in each case when the ship's head is directed to the points of the compass that were abeam while on the building slip, .e., occuring at that point where the disturbing force is perpendicular to the direction of the deflected needle; and, moreover, for the intermediate parts or points of the semicircle the deviation will be proportional to the sine of the azimuth of the ship's head, reckoned from the point of no deviation in the direction of the maximum. Or, again, to put it in another way, supposing the observer to be standing in the centre of the compass looking towards the direction of the ship's head when building, the semicircle to the right of him would have westerly, and that to the left easterly, deviation. The deviation caused by sub-permanent magnetism acquired in building, 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. The part due to sub-permanent magnetism remains the same in kind, though different in amount, in all latitudes and longitudes, 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, easterly 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.

268. Theoretical Representation of the Semicircular Deviation.-If we place a magnet before the compass with its S. end (blue pole) turned to the compass it will draw the N. end (red pole) of the needle to the ship's head, and as the ship is turned round there will be, in the first or eastern semicircle, a deviation of the north point of the compass to the right hand or east; in the second or western semicircle, a deviation to the left hand or west. This would produce one part of what is called the "semicircular" deviation.

If we place a soft iron rod vertically in front of the compass, with its upper end at the level of the compass, this end which will be S. (blue pole), will attract the north end of the needle, and produce a deviation of exactly the same kind as the magnet we have described above as having its blue pole pointing to the compass. It will, therefore, simply increase the semicircular deviation caused by the magnet.

If the N. end (red pole) of the magnet, or the lower end of the rod be nearest the compass-or if the magnet or rod be abaft the compass, an effect of the same kind, but in an opposite direction, will be produced.

A magnet to starboard or port of the compass will produce a similar effect to that already described, except that a deviation of one kind will be produced when the ship's head is in the northern semicircle, and of the other kind when in the southern semicircle. This is the other part of the "semicircular” deviation.

The effect of the two magnets and the one iron rod, which we have considered make up the whole of what is called the semicircular deviation.

269. 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 that 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.

Ship's head while building.

The north end of the compass needle on the poop or quarter deck is usually drawn.

The position in the ship from which the polar force may be considered as emanating may be illustrated by the following figure, representing eight iron ships built respectively on the eight principal points of the compass. Keeping in view the principle referred to in a former paragraph, that the N. end of the compass needle is attracted to that part of the ship which was south from the compass while building, we observed of the figures of the ships the position of the permanent south pole developed in the process of construction marked thus ●.

In the ship built head north it will be seen that the N. end of the compass needle is drawn to a point on the stern immediately in a fore-and-aft line; in the ship head N.E., to a point on the starboard quarter making an angle of 45°

from the fore-and-aft line; in the ship head East, to a point on the starboard side abeam of the compass; in the ship head S.E., to a point on the starboard bow at an angle of 45° from the fore-and-aft line; in the ship head south, to a point in the bow immediately in a fore-and-aft line; and so on for the several remaining directions.

Fig. 83 will further illustrate the way in which the permanent magnetism and the inductive magnetism of vertical iron acts upon the compass to produce semicircular deviation. Let it be supposed that the whole of the south polarity or attractive power of the above magnetism is concentrated in the point P on the port quarter of a ship built with her head near N.W. The ship is supposed to be swung round the compass, beginning at the N.W. point. The small circles represent the compass, the thick lines N ́S the compass needle, the dotted lines the magnetic meridian or the direction of the needle

when free from deviation. Beginning at N.W., and noting the position of the point P, it will be observed that there can be no semicircular deviation with ship's head in that direction, because the attractive force of the ship's magnetism at the point P is in a line with the compass needle NS. As the ship's head swings round towards the west, the relative positions of the point P and the compass needle will alter, and P will exert a pulling force upon the north end of the needle, causing it to deviate to the right from N to N', shown in the figure at West. The easterly deviation will increase until the ship's head swings to S. W., where it attains its greatest or maximum amount. After passing S.W. it gradually decreases past South until the ship's head reaches S.E., the opposite direction to that in which her head was built, where

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