it is again zero or nothing. The point P is now on the opposite side of the compass to what it was when her head was at N.W., but it will be observed that it is in a line with the needle, and can exert no deviating influence over it.

As the ship swings with her head towards the East, the needle will gradually be drawn to the left hand, until the westerly deviation attains its maximum at N.E. After passing N.E. the westerly deviation will decrease past North until the ship's head again reaches N.W., at which point there is no deviation. A very slight inspection of the figure will show that in the semicircle from N.W. round by the West to S.E. the deviation is easterly; while in the semicircle, or half the compass, from S.E. round by the East, the deviation is westerly. The above is merely given for the sake of illustration, but it must be remembered that no two ships are alike in their influence on the compass, nor will the ship's magnetism have the same effect on two compasses placed on different parts of the deck.

270. A soft iron mass and horizontal soft iron exert a wholly different influence on the compass from that hitherto described.

The effect of a soft iron ball on the needle when carried round the compass in the same horizontal plane is illustrated as follows (Fig. 84):—*

At the spherical ball of soft iron lies in the magnetic meridian, and North of the compass; it therefore, according to the law of like poles repelling and unlike poles attracting, produces no deviation, but the directive force of the needle is increased.

At 2 the sphere lies in the N.E. quadrant, and consequently, since the attraction is to the right, it produces E. deviation in that quadrant, and the directive force of the needle is increased.

At 3 the sphere lies East of the compass, and at right-angles to the deviation of the needle, where it produces no deviation, but still increases the directive force.

At 4 the sphere lies in the N.W. quadrant, and since the attraction is towards the left, it therefore produces W. deviation in that quadrant, and the directive force of the needle is increased.

With the sphere lying at 1a, 2a, 3a, and 4a, the effect on the compass will be similar to that of the respective positions 1, 2, 3, and 4; that is, producing no deviation at South and West, easterly deviation in the S.W. quadrant, and westerly deviation in the S.E. quadrant.

From the preceding remarks it is evident that—

Soft iron in the horizontal plane of the compass invariably increases the directive force of the magnetic needle.

Soft iron in the horizontal plane of the compass gives no deviation at the four cardinal points, N., E., S., and W., by compass; but the other position in each of the four quadrants are affected as follows:-if the iron lies in a direction

between N. and E. it produces Easterly deviation.

Westerly
Easterly
Westerly

Two soft iron masses on opposite sides of the compass, and in the same plane, affect the needle to the extent of their mass and force.

Two masses situated at right-angles with each other increases the directive force to the extent of their sum, and produce a deviation equivalent to the difference, of their forces.

271. How the action of a horizontal bar of soft iron revolving round the compass differs from that of a magnet. -If a bar of soft iron is held in the direction of the dip it becomes a magnet, the upper end of which, in North latitude, attracts the N. end of the magnet, if it is moved into a horizontal position, but still pointing North and South it is still magnetic, under the influence of the horizontal component of the earth's force, though in a smaller degree. If it is now moved horizontally in an East and West direction, its magnetism still further diminishes, and when it reaches the East and West position its magnetism disappears altogether. (See No. 231, page 321).

If, therefore, a compass is acted upon by this bar or horizontal rod of soft iron when directed towards the needle, and lying before or abaft it, the phenomenon is wholly different* from that arising from the action of a magnet bar, because, as we have just seen, the magnetism of the rod is not constant, but varies with the azimuth of the rod from the north. When, therefore, the bar is North or South of the needle, it attracts it strongly, but as the needle is already pointing North, its directive force is increased, but its direction is not disturbed; if the rod is carried radially, say to the East, the needle is drawn toward it and so has an Easterly deviation, which is a maximum when it is a little past N.E.

But the bar as it approaches the E. and W. position loses its magnetism; and when E. and W. has none, and therefore does not attract the needle, and

"The law of disturbance may be represented (for memory only) by this rule: the mass (or rod) attracts that pole of the needle which is nearest to it."-AIRY, A Treatise on Magnetism, page 138.

produces no deviation; consequently, the deviation will be zero when the bar is N. and S. and N. of the compass, zero when the bar is E. and W. and E. of the compass, and will have a maximum, as just stated, somewhere between N. and E.

It will be the same in each quadrant, except that in the second quadrant, E. to S., the S. end of the needle will be attracted by the N. end of the bar, thus giving a W. deviation. E. in the S. to W. quadrant and W. again in the W. to N. quadrant (see Fig. 85). It thus changes in each quadrant, and produces a deviation of the kind called quadrantal.

Fig. 85.

The top of the page represents the N.

272. Quadrantal Deviation is so named from its being easterly and westerly, alternately, in the four quadrants as the ship moves round a complete circle of azimuth. It is caused by the transient or inductive magnetism of horizontal soft iron, which in an iron ship is not alone the horizontal soft iron near the compass that produces it, but all connected with the hull, the keel, frame and fittings running fore-and-aft, athwartships, diagonally, over, under, and in the plane of the compass; in addition to which, in a steamer, there are the engines, screw shaft, &c. It is zero or nothing when a ship's head is near the North, South, East, or West points, and greatest on the quadrantal points. It is generally easterly in the N.E. and S.W. quadrants, and westerly in the N.W. and S.E. quadrants of the compass.

This quadrantal deviation is divided into two parts, one, which is the more important, and which has its maxima at or near the intercardinal points N.E., S.E., S.W., N.W., and the other, which has its maxima at the cardinal points N., S., E., and W., and which is generally so small that it may be neglected. It arises from soft iron unsymmetrically distributed.

To illustrate the way in which horizontal soft iron produces quadrantal deviation, let us suppose the whole of the induced magnetism in a ship to be Fig. 86. Magnetic.

represented by the soft iron bar B in Fig. 86. This cannot be so in actual practice, because the athwartship horizontal iron produces quadrantal deviation as well as the fore-and-aft iron, but we may suppose it may for the sake

of clearness. The small circles represent the compass, the thick lines within the small circles the compass needle, the dotted lines within the compass the magnetic meridian. Beginning at north, it will be observed that the bar B is parallel with the magnetic meridian, and will therefore be an inductive magnet while it is in or near that position, its after end, marked S., being a south pole; but as the bar B is in a line with the compass needle N, it cannot exert any deviating power upon the needle, either to the right or left. As the ship's head swings towards the N.W., the relative positions of the bar B and the needle N are altered, and the south end of the bar draws the north end of the needle to the left from N to N'. As the ship's head approaches the west, the bar B loses its polarity, and at west it is at rightangles to the magnetic meridian, and ceases to exert any influence on the compass. The ship's head now swings towards the S. W., and the bar B, as it turns towards the south pole, again becomes an inductive magnet; its after end being a north pole, and drawing the south end of the compass needle from S to S'. When the ship's head reaches south there is no quadrantal deviation, because the bar B is in a line with the compass needle. As her head swings towards the S.E., the needle is drawn from S to S', causing westerly deviation. At east there is no deviation, for the same reason that there was none at west. After passing east, the after end of the bar B becomes a south pole, and draws the north end of the needle to the right-hand in the N.E. quadrant. As the ship's head approaches the north, the quadrantal deviation gradually decreases until it becomes nothing at north. The reader will observe that the bar B in this case produces easterly deviation in the N.E. and S. W. quadrants, and westerly deviation in the N.W. and S.E. quadrants. Cases may arise where the deviation is westerly in the N.E. and S.W. quadrants, but they are very rare.

273. Constant Deviation.-The "constant" part of the deviation is (unless the compass be improperly placed), in all cases in which observations have been carefully made with good instruments, small in amount, and it probably more often arises from index or other instrumental error, or from error in the assumed direction of the magnetic north than from the action of the iron in the ship.

274. The quadrantal deviation, besides the remarkable uniformity in its zero points and in its direction in the several quadrants, has the further remarkable property, as before stated, of remaining unchanged in all magnetic latitudes, and of being little changed by the lapse of time.

275. The semicircular deviation differs from the quadrantal and the constant deviations in those respects. In all ships the semicircular deviation changes with a change of geographical latitude. In iron-built ships the semicircular deviation consists of two parts-one, the induced part which is generally the smaller, and varying (approximately) as the tangent of the dip; the other having its zero point when the ship's head or stern is on or near the point on which the ship's head was when building, and varying (approximately) inversely as the earth's horizontal force at the place. It will, therefore, diminish as we approach the line of greatest horizontal force, which does not differ much from the magnetic equator, and increase as we