proper place, and it is this difference which is called the declination or variation of the compass. The importance of the distinction between the magnetic and the true bearings will therefore be easily understood.

In a ship the iron used in its construction also affects the direction of the compass-needle, and in different degrees, and in different directions according to the eastward or westward position of the ship's head: for the methods of swinging ships, and separating this variable source of error from the variation, and of estimating its amount, the reader must be referred to the special article on magnetism contained in this series.

8. It is of more importance to the sailor to be able to discover the total effect of all the disturbing causes which influence the directive power of his compass. And the method he employs may be thus briefly described :-the magnetic direction of the sun, or some well-known star, is observed with the compass, and then its actual bearing is computed from the astronomical elements of the star's position at the time, and these are found in the 'Nautical Almanac.' A knowledge of spherical trigonometry is necessary in this, and almost every other astronomical problem. Next, the computed true bearing and the observed magnetic bearing are compared with each other, and their difference, if any exists, is the combined effect of the variation of the compass and the local deviation.

9. The direction in which the ship is moving is judged by the steersman, who observes what point of the compass coincides with a mark on the compass-box, placed there to indicate to him the direction of the foreand-aft line of the ship. Not that he believes the ship to be moving in the direction of her length, for she is under the influence both of the wind upon her sails, and of the resistance of the water through which she is moving; and between these two forces she makes a compromise, as it were, progressing nearly in the direction in which her head lies, but more or less to

the leeward, according to the strength of the wind, and the angle it makes with the side of the ship.

10. The amount of this deviation of the ship's track from the direction of the fore-and-aft line is called leeway, and experience, that wary and ancient teacher, is the seaman's guide in estimating its amount in the various circumstances in which he is placed. This then is sometimes another allowance he has to make upon the indications of his compass; nor is this all, for the whole body of the water may be moving and drifting the ship with it, without altering in the least her apparent course. If the current is running in the same direction as that in which the ship is urged by those forces already mentioned, no alteration of the ship's course will ensue, nor indeed, if the current should run in a direction diametrically opposed to this: the only effect then is, that of increasing or diminishing the rate of the ship's motion by the whole amount of the rate or drift of the current, as it is called.

If, however, the setting of the current cross the track which the ship would have pursued without its interference, then both the course of the ship and its rate will be affected by this new disturbing force, in a degree proportioned to its strength.

B

11. In the annexed diagram (fig. 6) let A B be the direction and distance which the ship would have gone supposing C no current had existed, and let BC be the direction and the drift of the current in the same period of time, then a c will represent the actual track of the ship under these circumstances, and it will be at once seen that her rate has been increased, and her course made more westerly by the westerly run of this cur

rent.

W

E

Fig. 6.

S

This will serve to show how an allowance can be

made for the effect of a current whose direction and rate are known, when the rate also at which the ship is moving through the water has been ascertained.

12. What better method could be devised for reckoning the distance you run upon the ground, than that of thing a measuring tape to a log, and then, while running, letting the line slip lightly through the hand, so that the log may not be displaced, nor your line broken? Now this is just the method in ordinary use A piece of wood of the shape depicted in the

at sea.

margin is attached to a long line, divided by certain marks into lengths called knots, each equal to of a nautical mile. The log is thrown overboard, and being weighted at its curved edge, stands perpendicularly in the water with its face towards the ship, and protected from the action of the wind which is urging the ship onwards: the line is then allowed to slip lightly out for 30 seconds or part of an hour, and the knots which have passed being reckoned, tell how many miles per hour the ship is advancing through the water. The length of the knots should be examined from time to time, as well as the sand-glass with which the time is measured, and if either or both should be found to be inaccurate, a correction must be calculated and applied to the estimated rate.

13. The compass and the log are the instruments by which the common journal of a ship's voyage is carried on; from hour to hour her compass, course, the wind, and the rate of the ship, with any other particulars worthy of note are entered on the log-board, and every day at noon a reckoning is made of her progress, and of her place upon the world.

The compass and the log. How many weak places have been indicated in the thread which they weave between the sailor and his home! The continuous zig-zag path which by their aid the sailor marks upon

his sea-chart, if not checked by other means, would perpetuate any error in the reckoning which might be accidentally, inadvertently, or carelessly made; every error of observation, of calculation, of instrumental imperfection, or of ignorant interpretation, by any one to whom the watch might be intrusted. The captain's skill would be set at nought: indeed it would be but a name-a sound signifying nothing. How can he assure himself that all has been well in the few hours which nature has demanded of him for the restoration of his physical power and mental health by sleep? The steering of the ship is intrusted in rotation to his men : are they all careful? are they all faithful? Unsuspected currents, too, may have swept the ship from her intended track, and, as we have seen, may have falsified the statements of our two silently-eloquent watchmen, the compass and the log.

LESSON IV.

THE SEXTANT, THE CHRONOMETER, AND THE NAUTICAL ALMANAC.

1. It is now that the captain must avail himself of such astronomical skill as he may possess: it is now that he has an opportunity of asserting the superiority of his scientific training over that of the ignorant and boastfully confident man who "trusts that all will be well with him, and invokes, nay claims as a right, the protection of that bountiful Providence whose instructions he indolently and arrogantly refuses to receive.

But, for our captain, the sun, the moon, the stars in their eternal round are enlisted with him in his labour; he compares the feeble, wavering efforts of poor humanity with the great truths written in the heavens, and the light of a thousand suns makes clear his way across the pathless sea.

2. In order to avoid unnecessary repetition, it must

be assumed that the reader has carefully studied the articles on astronomy contained in this series: and that he has familiarized himself with the technical terms in common use in this branch of science; that he knows that the declination of a heavenly body means its angular distance north and south of the celestial equator. And that this element of the position of the heavenly bodies, in the case of the sun is subject to an annual fluctuation between 234° north and 234° south, as a consequence of the inclination of the axis of the earth to the plane of the orbit in which it moves. He will have learnt little indeed if he does not know that this is one of the many beneficent ordinations for which his daily thanksgivings are due: that it is this which produces seed-time and harvest, and on the regular periodicity of these changes depends our "daily bread."

3. The Nautical Almanac' is a book issued annually by the British government, for the special use of astronomers and navigators; and contains predictions as to the places of a large number of the sailors' celestial monitors for several years in advance; so that the sailor who is starting on a long voyage may not be embarrassed for the want of such important information if the period of his absence should run over two or three years.

4. In the annexed diagram (fig. 7)

the

globe is intended to represent the earth, P and Q the north and south poles. C the middle point or centre of the earth, c the situation of the ob

Fig. 7.

server, C S and

c S lines point

ing to the star S which is vertically over a, a being,