15.—It vanishes at near East and West courses, and it is greatest at near North and South courses.

16.-The North point is generally drawn to the weather side. The effect is to throw the Ship to windward on Northerly courses, and to leeward on Southerly courses.

17. Keep away on Northerly courses, and keep closer on Southerly

courses.

18.-No. In the Southern hemisphere the rule is frequently reversed, but in order to determine it, observations must be made in every ship.

19.-Draw two chalk lines on the deck, at right angles to each other, one fore and aft, the other athwartships, and with their point of intersection under the centre of the compass needle.

Look from the South to North, or from North to South.

BRING THE SHIP'S HEAD EITHER NORTH OR SOUTH, CORRECT MAGNETIC. Place a magnet, or, if necessary, two, on the deck, athwartships, either before or abaft the binnacle, with its centre on the fore and aft line.

If the North end of the needle deviate to the right hand, place the North end of the magnet to the right hand.

But if it deviate to the left, place the North end to the left.

Then move the magnet gradually towards the binnacle, or from it, till the needle points correctly.

Next, BRING THE SHIP'S HEAD EAST OR WEST, CORRECT MAGNETIC. Place another magnet on the deck, fore and aft, at either side of the binnacle, with its centre on the athwartship line.

If the North end of the needle deviate to the right hand, place the North end of the magnet to the right hand.

But if it deviate to the left, place the North end to the left.

Then move the magnet towards the binnacle, or from it, till the needle points correctly.

The preceding processes correct the Semi-circular Deviation.

To correct the Quadrantal Deviation, proceed as follows:

The Binnacle being fitted with cast iron correctors, or with small brass boxes, one on each side, port and starboard, of the needle, and at the same height as it.

Then bring the Ship's head on one of the Quadrantal points, N.E., S.E., S.W., or N.W., correct magnetic.

Then move the correctors towards the needle or from it, or put small chain into the boxes, till the needle points correctly.

In the above diagram, let the two lines at right angles to each other represent the cardinal points of the correct magnetic compass, and let the other parts of the diagram represent the deck plans of ships with their heads correct magnetic North, South, East and West.

One of the ships at the upper part of the diagram has her head North, the other has her's South.

In each ship, let the fore and aft line represent the correct magnetic meridian.

Look from South to North, or from North to South.

Let the dotted line represent the compass needle with its North end marked N, drawn to the right hand by the iron in the ship.

Let the athwartship lines represent the correcting magnets, one before the binnacle, the other abaft it.

Then, because similar poles repel, and opposite poles attract each other (Page 458) we have the following rule:

Place the upper magnet, with its North end, marked N, to the right hand, and it will repel the North end of the needle, while the South end of the magnet will attract the North end of the needle, and if the magnet be brought sufficiently near, these repulsions and attractions will force the needle into the correct magnetic meridian. The lower magnet will have the same effect.

When the ship's head is East or West, the compass needle is then athwartship, and the correcting magnet must be placed fore and aft.

By all the figures it appears that, when looking from the South towards the North, whether the ship's head be North, South, East or West.

If the North end of the needle deviate to the right hand, place the North end of the magnet to the right hand.

But if it deviate to the left, place the North end to the left.

The rules in the preceding page may be expressed as follows: Whether the ship's head be North, South, East, or West, correct magnetic.

Place the magnets in the direction of East and West, correct magnetic; and so that similar poles may repel, and opposite poles attract, the compass needle into the direction of the correct magnetic meridian.

LEADING LIGHTS.

ENGLISH CHANNEL.

ENGLISH COAST.

Inter. denotes Intermitting. Rev. denotes Revolving. L.S. means Light Ship.