leave it on either side. If the two parts varied exactly, and not merely approximately, according to these laws, we might, by observations made in two different latitudes, discriminate the two parts, and might predict the changes the deviations would undergo in different latitudes.

TO FORM A TABLE OF DEVIATION.

276. To Make a Table of Deviations.-There are several methods in use for forming a table of deviations. When the ship's head is brought to each point of the compass in succession this is called swinging her, and care must be taken to see that she is beyond all magnetic influence from surrounding objects, and that the vessel herself is equipped for sea.

277. When in port there are two principal methods in general use for finding the deviation, viz. :-Method I, by the known correct magnetic bearing of a distant object, and Method II, by reciprocal simultaneous bearings, i.e., with a compass on board and a compass on shore.

278. METHOD I.-By the known bearing of a distant object. -The requisite warps being prepared, the ship is to be gradually swung round so as to bring her head successively upon each of the 32 points of the Standard Compass; and when the ship and the compass card are perfectly steady, and her head exactly on any one point, the direct bearing of some well-defined object is to be observed with the Standard Compass, and registered. The ship's head is to be gently warped round in the same manner to the next point, and when duly stopped and steadied there, the bearing of the same object is to be again set, and again recorded; and so on, point after point, till the exact bearing of the one object has been ascertained with the ship's head on every separate point of the compass.

279. The object selected for this purpose should be at such a distance that the diameter of the space through which the ship revolves shall make no sensible difference in its real bearing, and should not exceed the onehundredth part of the distance of the object. The distance must depend on the range the ship takes when swinging; if she be at anchor, in a tide-way, from 6 to 8 miles is not too much; brought up by the middle (in a dock) 2 miles will suffice.

280. The next step is to determine the correct magnetic bearing of the selected object from the ship; or, in other words, the compass bearing it would have from on board if it were not disturbed by the attraction of the iron in the ship. This is effected by taking the compass to some place on shore (avoiding local influence) from which the part of the ship where the compass stood and the object of which the bearings had been observed shall be in one with the observer's eye, or else in the exactly opposite direction. The bearing of the object from that spot will evidently be the correct magnetic bearing from the ship by the compass. The difference between the correct magnetic bearing of the object and the successive bearings which were observed with the compass on board, when the ship's head was on the several

points, will show the error of each of these points which was caused by the ship's iron; or, in other words, the Deviation of the Standard Compass according to the direction in which the ship's head was placed.

(b) The correct magnetic bearing of the distant object will be the mean value of all the observed bearings, if observed on equidistant points; or of four or more compass bearings, if taken, also, on equidistant compass points.

281. II. By reciprocal bearings.-Should there be no suitable object visible from the ship, and at the requisite distance as stated above, the deviations must be ascertained by the process of reciprocal bearings. A second compass is placed on shore where it will be entirely beyond the influence of iron of any description, and where it can be distinctly seen from the Standard Compass on board. Then take, simultaneously (known by pre-concerted signal), the bearing from each other of the compass on shore and the compass in the binnacle, as the ship is warped round so as to bring her head successively upon each of the thirty-two points of the Standard Compass on board, or on each alternate point.

To ensure the success of this operation, the compass on shore should not be more distant from the ship than is consistent with the most distinct visibility with the naked eye, of both compasses from each other. The observations should be made as strictly simultaneous as possible, the time at which each bearing is taken being noted both on shore and on board. It will be found convenient in practice for the shore observer to chalk each observation on a black board, to be read at once from the ship, in order that the observation may be repeated if any apparent inconsistency presents itself.

Before this process is complete, the Standard Compass should be carried on shore, in order to be compared with the compass used there, by means of the bearing of some distant object, and the difference, if any, is to be recorded; and in all cases, when compasses are compared, the caps, pivots, &c., should be first carefully examined. The shore compass gives correct magnetic bearings.

The difference between the correct magnetic bearing of the Standard Compass as observed from the shore, and the bearing of the shore compass as observed from the ship, with her head in any particular point, reversed, i.e., with 180° added or subtracted, will show the error on that point which was caused by the ship's iron; in other words, the deviation of the Standard Compass according to the direction in which the ship's head was placed.

282. III.-By Marks on the Dock Wall.-This is a very convenient method where it can be practiced. At Liverpool the correct magnetic bearings of the Vauxhall chimney, from various points of the dock walls, are painted in large figures on the walls, so that the bearing of the same chimney may be observed as the ship swings with the wind and tide; and at the same time that bearing marked on the wall, which is on a line between the Standard Compass and the chimney, is noted.

The difference between those bearings is the deviation for the point on which the ship's head is at the time.

In a similar manner, at Cronstadt, the correct magnetic bearings of a conspicuous point on a public building are painted on the mole.

283. If, during the operation of swinging, a haze obscures the shore compass, while the sun at the time is shining brightly, a number of points may be secured by time azimuths, which otherwise might be lost. Timeazimuths are also advantageous where the second of the above methods cannot be used for want of an assistant observer for the shore compass; and when the first of the above methods is not available owing to the length of the ship and the scope of the mooring, combined with the most distant objects in sight not being sufficiently far off to render the difference of their bearings insensible as the ship swings round to the tide. In such cases Godfray's Azimuth Diagram, as also Azimuth or Sun's True Bearing Tables, computed for intervals of four minutes, by Staff-Commander J. BURDWOOD, R.N., published by the Admiralty, will be found useful as superseding the calculation for the determination of the True Azimuth.

284. Commander Walker, R.N., has shown* that the deviation may be ascertained with sufficient accuracy by selecting a distant object, as before, "and as the ship swings by wind or tide from one point to another, write down the compass bearings of the distant object opposite the direction of the ship's head. As the ship swings round there will be two nearly opposite points of the compass on which the bearings of the distant object agree, and this should be the correct magnetic bearing of the object." The deviation is then found as in the first method.

285. The Dumb Card.-The difficulty of finding the correct magnetic bearing of the ship's head may be obviated, however, by using the dumb-card, i.e., a compass card without the needle, slung in gimbals, with its centre over a fore-and-aft line of the vessel, and as near to its middle as possible. The card is fitted with sight vanes, similar to an azimuth compass. Having obtained the correct magnetic bearing of a distant object, place the card so that it shall point out that direction, and screw the sight vanes to the card, so as to cut the object with the thread. Then, as the ship is swung, the card must still be kept pointing out the correct magnetic bearing of the object by means of the sight vanes, and where the fore-and-aft line meets the edge of the card, must then be the correct magnetic bearing of the ship's head.

286. IV.-By Observations of Amplitudes and Azimuths.-The errors of the compass obtained from observations of celestial objects always contain variation and deviation, and may also be affected by local attraction if the ship be near any disturbing cause. The True Bearing

is found by calculation of an Amplitude (pages 198-204) or Azimuth (pages 252-259), and by comparing this with the bearing of the celestial object shown by the compass the correction or error of the compass is found for the • Magnetism of Ships and the Mariner's Compass.

ZZ

direction of the ship's head when the observation was taken. Variation or declination charts give the amount of error due to that cause; and the difference between the variation and the compass error is the deviation. By continuing the process to the eight principal points of the compass a complete table can be found by Napier's Graphic Method as explained at pages 365-369.

287. To Name the Deviation.-Rule.-When the reading by the shore compass (reversed), or the correct magnetic bearing of the distant object, is to the right of the reading by the compass on board, the deviation is easterly; when to the left, westerly.

Thus, suppose the correct magnetic bearing from the shore compass, with ship's nead at N.W., is N. 15° E., and the bearing of shore compass from the ship is S. 11° W.: to find deviation proceed thus:

Reverse of the bearing by shore compass S. 15° W.

or correct magnetic bearing

Bearing from ship

Deviation

S. II W.

4 E.

When the ship's head lies N.N.E., let the binnacle compass bearing of the shore object or compass be N. 19° 30′ E., and the bearing of the binnacle compass from the shore compass be S. 27° 0' W.: required the deviation.

The opposite point to S. 27° 0′ W. is N. 27° 0' E., which is 7° 30' to the right of N. 19° 30' E. Hence the deviation is 7° 30′ E.

288. The directions of the ship's head having been taken by the compass in the ship, are therefore affected by the local attraction, and the apparent compass bearing of the ship's head differs from the correct magnetic bearing by the amount of the local deviation due to the position of the ship. For instance, when the ship is apparently lying with her head east, it is not the true magnetic east; but supposing the local deviation to be one point easterly, the east point of the compass card will be drawn to E. by S., the true magnetic direction of the ship's head will be E. by S.

and

The observations and tabulated results are incomplete until the correct magnetic bearing of the ship's head at each observation is found.

289. The following shows the arrangement of tabular forms for finding the deviation by the several processes described.

I. By Bearing of a Distant Object.

Correct magnetic bearing of distant object from ship N. 63° o' W., distant II miles.

290. The seaman must remember that the corrections thus obtained belong to the compass by which the observations are made, and to that compass while it is in its proper place, and that these corrections will furnish no guide whatever to the effects of the iron on a compass placed in any other part of the ship; but if, while swinging, the direction of the ship's head by the other compasses is noted and tabulated, the deviation of all the compasses can be found.

291. It is possible to correct mechanically the compass of an iron vessel in such a manner that the deviations on all courses may be very much reduced or even destroyed altogether.

292. Necessity of Compensation.-The object of applying magnet bars to correct the larger deviations of the compass in iron ships is two-fold: first, the reducing the larger semicircular deviations, which are embarrassing when they exceed a certain amount; and secondly, and more especially, the equalizing of the directive force on different courses. The principles on which their application depends are obvious, for it has been shown that all the polar forces acting in a ship may be represented by an imaginary magnet, and it is therefore certain, from well known laws of magnetic action, that we can neutralise the effect of these disturbing forces by introducing a real magnet whose force has the same magnitude but acts in an opposite direction.

Compensation by magnets does away with one of the errors to which an uncompensated compass is liable. There are changes to which the magnetic condition of an iron ship is subject, but in addition to these sources of error an uncompensated compass changes its deviation with any alteration that occurs in the amount of the earth's horizontal force, and this takes place when the ship changes her station on the earth's surface; for instance, if from sub-permanent magnetism in the Bay of Bengal an uncompensated compass had a deviation of ten degrees, in the Gulf of St. Lawrence it would have a deviation of thirty degrees.

If the deviations are large in amount embarrassments are produced, since the angular movement in azimuth of the ship's head is not represented by a

The time-as taken by compared watches-may be omitted if the shore observations can be clearly made out by being chalked on a black board.