of which for its various points must be ascertained by what is called swinging ship for the errors of the compass.

When a ship is "close hauled," that is, when she is sailing as near to the direction of the wind as she can be brought, that part of the wind which acts upon the hull and rigging, together with a considerable part of the force exerted on the sails, tend to drive her immediately from the direction of the wind, or, as it is termed, to leeward. But since the bow of a ship exposes less surface to the water than the side, the resistance will be less in the fore and aft direction than in the athwartship direction; the velocity, therefore, in the direction of her head will, in most cases, be greater than the velocity in the direction of her side, and the ship's real course will, on the basis of the composition of forces, be between the two directions: the effect produced is a continual drifting of the vessel from the wind with the head still in the same direction as if no drifting took place, and consequently the compass showing the same course; but if the ship drifts in this manner, her keel will make a streak or wake in the water in a direction opposite to the point towards which she is moving. The angle contained between the line of the ship's apparent course and the line she really describes through the water, is termed her leeway-which may be also expressed as the angle which the ship's keel makes with her actual path through the water.

The amount of leeway to be allowed will depend upon a variety of circumstances, as the build and trim of the ship; the force of the wind; the quantity of sail carried; the rate through the water; the sails being properly set to receive the action of the wind, etc. : hence no general rules can be laid down for estimating it.

With the wind aft or on the quarter there can be no leeway. A ship close hauled, wind moderate and sea smooth, makes no leeway. In heavy weather with small sail on, there is often considerable leeway. For a ship when she lies-to, observe the points to which her head comes up and then falls off, and take the middle point for the apparent course on which to allow the leeway.

The only method that ought to be relied on, in practice, of ascertaining the amount of leeway, since it is a correction of the apparent course, is that of actually measuring the angle before mentioned. This can be done as follows draw a small semicircle on the taffrail, with its diameter inwards and at right angles to the ship's keel; divide the semi-circumference into 16 points, and their halves; then observe the angle contained between the semidiameter which points right aft, and that which points in the direction of the wake, and it will be the angle of leeway required. But the most accurate method of determining the leeway is to have a semicircle drawn on the taffrail, as before described, with a low crutch or swivel in its centre. Then, after heaving the log, the line is to be slipped into the crutch just before it is drawn in, and the points and quarters contained between the direction of the log-line and the fore and aft line of the semicircle will be the quantity of leeway.

When the ship makes leeway the direction of her head by compass is only the apparent, not the real course; and the amount of leeway is always. allowed to the apparent course in a direction from the wind; hence, looking towards the ship's bow

RULE.-Wind on starboard side, allow leeway to the left hand.

Wind on port side, allow leeway to the right hand.

Example.-Ship's head by compass N.N.W., leeway 1 points, with wind N.E., gives the actual direction of her course N.W.

N.

Example. Ship's head by compass W.S.W., leewaypoint, with wind south, gives the actual direction of her course W.S.W.

W.

Example. Ship's head by compass south, leeway 2 points, with wind W.S.W., gives the actual direction of her course S.S.E.E.

These examples will make the method of allowing for leeway clear; but remember that leeway is not an error of the compass, only a correction to be applied to the course under special circumstances, and is independent of direction of ship's head as to amount.

Correct the following courses for leeway :

COURSES.-There are three different kinds of courses, each of which has its special corrections, to be applied in a special way, when converting one course into another.

I. The COMPASS COURSE is the angle that the ship's fore and aft line makes with the axis of the compass needle, which should coincide with the N. and S. line of the compass card.

This course will be affected by the deviation due to the direction of the ship's head, and the variation shown by the chart as appertaining to the locality where the ship is being navigated. If there is deviation, its application reduces the compass course to the magnetic course; and the further application of the variation to the latter gives the true course.

II. The MAGNETIC COURSE is the angle that the ship's track makes with the magnetic meridian.

This course is affected only by the variation, the application of which converts it into a true course. When there is no deviation, the compass course will be the magnetic course, but it must not be rashly concluded that because a ship is all wood there is no deviation. There are sure to be iron fittings which may affect the compass.

III. The TRUE COURSE is the angle that the ship's track makes with the terrestrial or true meridian.

This is the true course made good and is derived from the course steered. Since the latter does not necessarily, and at all times, coincide with the correct compass course, it follows that the true course made good must be obtained through the application of the corrections for leeway, deviation, and variation-any or all of them, as required; the true course is the

basis of the ship's position by dead reckoning from day to day, or at any intermediate time.

PRELIMINARY AND CAUTIONARY OBSERVATION ON THE CORRECTION OF COURSES.—Let it be clearly understood, and without the necessity for further repetition, that since the compass is the representation of the visible horizon which bounds the observer in every direction at sea, he must never lose sight of the conception that the position of the ship, and hence his own as navigator, is the centre of the compass card. This is most important, for, in speaking of the corrections of the various courses, these corrections now it may be to the right, and now to the left, bear reference entirely to that central position, with the navigator looking in the direction of the point towards which the ship's head is directed.

Hence the significance of the terms right and left, as compared with the movements of the hands of a watch, is this-right is with watch-hands, left is against watch-hands.

The Magnetic Course and the Variation being given, to find the True Course

Variation of the compass is the angle that the magnetic meridian makes with the true meridian. For any given place it affects every compass course and bearing alike, to the same amount and in the same direction; thus, off the Isle of Wight, where the variation is 16° W., the compass needle trends 16° westward of the terrestrial meridian, so that the Pole Star bears by compass N. 16° E., which is the compass bearing corresponding to true north. In that vicinity every compass course and bearing is affected to the extent of 16° W.

RULE. Variation westerly, allow it to the left of the magnetic course. Variation easterly, allow it to the right of the magnetic course. This rule you can often carry out by looking at a compass card, and making the correction mentally, but not always; you should aim, however, at getting an accurate result without reference to the compass card.

To correct a Compass Course steered, for the Deviation Take the deviation for the given course from the Deviation Table (page 339); then, to get the correct magnetic course

RULE.-Deviation westerly, allow it to the left of the compass course. Deviation easterly, allow it to the right of the compass course.

The modern practice is to use the algebraic sum of the variation and deviation, which is then called the compass error. For example, variation 15° W., deviation 10° E., compass error will be 5° W., which apply as before, E. to the right, W. to the left.

The diagrams should assist materially.

The westerly correction it will be observed is + from N. to W., - from W. to S., from S. to E., and from E. to N.

The easterly correction is the opposite.

When the variation, deviation, or compass error is additive, if the sum exceeds 90° take it from 180°, and the remainder will be S. if previously N., but N. if previously S.; also, when the variation is subtractive but exceeds the course, subtract the course from the variation, and name the remainder E. if the course was W., but W. if the course was E.

Modern practice tends to abolish points and quadrantal degrees in the compass and to treat it as a circle of 360°, north being o° or 360°, E. 90°, S. 180°, and West 270°.

By turning quadrantal compass courses into their equivalent value reckoned from north right round to 360° by table of compass equivalents all easterly errors become plus and all westerly errors minus, and all leeway when on the port tack is plus and on the starboard tack minus. After correcting the course it is easily turned into its proper quadrant by the same table. This is by far the best method.

The excess of 360° would be shown as from o° towards 90° and the defect as from 360° towards 270°. For example, compass course 350°, error 20° E., 350° + 20° = 370° = 10°, that is 10°, that is N. 10° E. Again, compass course

=

5°, error 30° W., 360° + 5° = 365° — 30° = 335° or N. 25° W.

Examples in the Correction of Courses

As shown above, a course may be affected by three things-variation, deviation, leeway.

An algebraical sum can be made of the variation and deviation; the leeway is applied separately.