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This, then, is the semicircular deviation, and its total effect is that which has just been considered.

But since the ship's force generally acts in & direction making an angle with the axial line of the ship, it has been deemed more convenient to substitute, for this

B single disturbing force, two disturbing forces -one acting fore and aft (represented in the formula p. 740, by the coefficient B), and the other acting athwartship (represented by the coefficient C). See also, Fig. 9.

Fig. 9. That part of the semicircular deviation represented by the coefficient B may be experimentally * illustrated as follows:

A magnet placed before the compass with its S. end (biu pole) directed towards the compass, will draw the North end (red pole) of the needle to the ship's head, then, as the ship is turned round there will be, in the Eastern semicircle, a deviation of the North point of the compass to the right hand or East; in the Western semicircle, a deviation to the left hand or West.

A soft iron rod placed vertically in front of the compass, with its apper end at the level of the compass, this end, which will be a blue pole, will attract the North end of the needle, and produce a deviation of exactly the same kind as the magnet described abore 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.

For the other part of the semicircalar deviation, represented by

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* You require the form of a ship's deck cut out of paper, a pocket cum. pass 11 to 2 inches in diameter, two small magnets about 2 or 3 inches long, and a soft iron rod; these you can so manipulate as to illustrate the phenomena of a ship built in any direction, if you previously determine the cardinal points in respect to the table on which you place your imaginary ship, and move the latter on a centre.

the coefficient C, place a magnet to starboard or port of the compass ; it will produce an effect similar to that already described, except that a deviation of one kind will be the result when the ship’s head is in the Northern semicircle, and of the other kind · when in the Southern semicircle.

The effect of the two magnets and the one iron rod make up the whole of what is called the semicircular deviation ; for which

d = B sin u + C cos z B being the semicircular deviation on the East course or point

by compass, and C the semicircular deviation on the North course.

It may also be noted here that the sign + is used for East, and the sign - for West.

Each of the coefficients B and C may have the signs plus and minus, as significant of the direction in which the ship was built, and the character of the semicircular deviation produced therefrom.

B is the representative of a force acting fore and aft.

+ B indicates that the ship was built with the head in some Southerly direction, and that the ship's force attracts the N. (red) end of the needle towards the head, the consequence of which is that, in respect to the compass, it produces E. deviation in the Eastern semicircle, and W. deviation in the Western semicircle, as shown by the signs in Fig. 10; also with the maximum at East and West.

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- B indicates that the ship was built heading in some Northerly direction, and hence has a force attracting the N. end of the needle towards the stern, from wbib it results that, in

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respect to the compass, there is W. deviation in the Eastern semicircle, and E. deviation in the Western semicircle, as shown by the signs in Fig. 11; also with the maximum at East and West.

C is the representative of a force acting athwartship.

+ C indicates that the ship was built with the head in some Easterly direction, and that the force attracts the N. end of the needle towards the starboard side, giving E. deviation in the Northern semicircle of the compass, and w. deviation in the Southern semicircle, as shown by the signs in Fig. 12; also having a maximum at North and South.

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C indicates that the ship was built heading in some Westeriy direction, and hence has a force attracting the N. end of the needle towards the port side, giving W. deviation in the Northern semicircle, and E. deviation in the Southern semicircle, -as shown by Fig. 13; also having a maximum at North and South.

Hence the distribution of each coefficient of the semicircular deviation, according to the direction of the ship's head when building, is as follows:Ship built head North gives B N.E.

B and + c
East
S.E.

+ B and + C
,i
South + B
+ B and

C
West
N.W. - Band - C

+ C

S.W.

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Thus far the semicircular deviation has been spoken of as if it were easy to find, unchanging, and easily corrected if so required. But unchanging it is not.

You have already been told that the earth's horizontal force is greatest near the magnetic equator, and decreases when proceeding Northward or Southward from that line; also that the dip of the needle (and hence the vertical force) is the reverse of this,being nil on the magnetic equator and increasing when proceeding Northward and Soathward in the direction of the earth's magnetic poles, and what is more, with a different pole of the needle downwards in the two hemispheres.

Now the coefficients B and C, which give the semicircular deviation, are each made up of two components, viz., (1) the subpermanent magnetism of the hard iron, and (2) the transient magnetism induced in the soft iron by the earth's vertical force. The first produces a semicircular deviation inversely proportional to the horizontal force at place; the meaning of which is that, though there is no change of name, there is decrease in amount with increase of the earth's horizontal force, and vice versa. The second produces a semicircular deviation proportional to the tangent of the dip; the meaning of which is that not only is there change of amount-decrease with decrease of vertical force, and increase with increase-but, on opposite sides of the magnetic equator there must be change of name. It does not, however, follow from this that the deviation as a whole must change its name, which will depend upon the relative proportion of subpermanent to transient induced magnetism.

When B and C are uncorrected, it is easy to understand that a deviation card has but a limited value, and that the change of deviation due to change of magnetic latitude requires to be constantly checked by observations of the heavenly bodies : and this remark applies with equal force to a compass where B and C have been corrected with permanent magnets, since it is impossible that these can compensate a changing element.

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

Note the effect of a soft iron ball* on the needle when carried round the compass in the same horizontal plane. See Fig. 14.

At 1 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, it therefore, since the attraction is towards the right, gives 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 direction of the needle, where it gives no deviation, but still increases the directive force.

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FIG. 14. At 4 the sphere lies in the N.W. quadrant, it therefore, since the attraction is towards the left, gives W. deviation in that quadrant, and the directive force of the needle is increased.

With the sphere at 1", 2", 34 and 44, the effect on the compass must be similar to that of the respective positions 1, 2, 3, 4; that is, giving no deviation at South and West, E. deviation in the S.W. quadrant, and W. deviation in the S.E. quadrant.

* An iron sphere is really, as already indicated, slightly magnetic in the direction of the dip; and so must be elongated iron correctors; the illez tration nevertheless holds good.

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