DESCRIPTION OF AZIMUTH TABLES. One of the best methods of determining the deviation is to apply the sun's true bearing or azimuth to the sun's bearing by compass, corrected for variation at the position of the ship. Davis's azimuth tables are computed between the parallels of latitude 30° N. and 30° S. inclusive, while those by Burdwood take in from 30° to 60°. They are identical in arrangement, and one description applies to both The azimuths are not given when the altitude of the sun exceeds 60°, as greater dependence can be placed on the observation when the body has a low altitude. The time and bearing given in the tables refer to the sun's centre; but as, from the effect of refraction, the sun's centre at rising or setting is elevated about 33 minutes. above the horizon, it becomes necessary to take its bearing when the lower limb is 16 minutes, or the semidiameter, above the horizon. In determining the deviation it is obvious that the difference between the sun's true bearing, or azimuth, and the sun's bearing by compass will give the variation for that position of the ship's head. The deviation will therefore be the difference between the variation thus observed and the variation obtained from the "Chart of the Curves of Equal Magnetic Variation." This chart is a most important adjunct to navigation, as it not only supplies, by inspection, the variation due to any geographical position on the globe, but it shows at a glance those parts of the ocean where great differences of variation occur, within limited areas; for example, in running up the English Channel from the westward-say from the neighbourhood of Cape Clear to the Downs (a distance of about 480 miles), the variation changes 6 degrees, or fully half a point, which necessitates watchfulness and periodical alterations in the courses steered. DIRECTIONS FOR USING THE TABLES. Turn to the pages indicating the latitude of the ship, then under the column of declination and along the line of the object's hour angle find its true bearing, or azimuth, reckoned from the elevated pole, viz., from north in latitude north, from south in latitude south, and towards east or west, according to instructions at the foot of each page. Thus (Burdwood, page 179), in latitude 47° North., with sun's declination 18° S., at 9h. 24m. A.M. the sun's true bearing is N. 141° 27′ E., or at 2h. 36m. P.M. it is N. 141° 27′ W. If the compass bearing requires it, this can be reckoned from the depressed pole by subtracting from 180°, so that N. 141° 27′ E. becomes S. 38° 33′ E. Example 7th October, 1880. Steering East. ... Latitude, 49° 0' Ν. Declination,......... 5° 45' S. Bearing of sun by standard compass,...... S. 39° 30′ E. Or, reckoning from the elevated pole,..... N. 140° 30′ E. Variation, as shown on the chart (1880),.. 22° 50′ W. Required, the Deviation of the Standard Compass. Sun's true bearing by tables,... N. 122° 20′ E. N. 140° 30′ E. Sun's bearing by compass, Variation observed in ship,.. 18° 10′ W. Variation from the chart,.. 22° 50′ W. Deviation with ship's head East, 4° 40' E. NAPIER'S DEVIATION DIAGRAMS. The following method of finding the deviations of the compass is due to James R. Napier, F.R.S., and consists of two parts, namely, the printed diagram and the curve which is filled in by hand. It has the advantage of being equally useful whether the observations have been made on equidistant points or not. The diagram consists of a vertical line of about 18 inches in length, divided into 32 equal parts, representing the 32 points of the compass, commencing at the top with north. The line is also divided into 360 equal parts representing degrees, which are numbered from 0° at the top to 360° at the bottom. These divisions are also numbered, according to the usual mode of dividing the circumference of the compass card, from 0° at north and south, to 90° at east and west. The line may, in fact, be considered as the margin of a compass card cut at the north point and straightened. The vertical line is intersected at each of the 32 points by two straight lines inclined to it at angles of 60°, one a plain line dipping to the left, the other a dotted line dipping to the right. The least number of observed deviations available for obtaining a complete curve is the deviation on 4 points distributed equally, or nearly so, round the compass; but, if possible, the deviations should be observed on 8 or more points. If the deviations are observed on 4 points only, these should be at or near N.E., S. E., S. W., and N.W. The points next in importance are North, East, South, and West. Cases may also occur in which by the ship swinging round at her anchor in a tide-way, or to the wind, or by the aid of a steam-tug, the deviations may be observed on various directions of the ship's head, not being necessarily exact points of the compass; or similarly whilst under steam or sail at sea, a number of azimuths of the sun may be observed, and hence the deviation obtained. In these cases, the method here described furnishes a ready and effectual mode of obtaining a result in which the errors of individual observations are as far as possible compensated, and any egregious errors eliminated. The observed deviations are to be laid down on the diagram in the following manner,-take, on the vertical line, one of the compass courses on which an observation has been made, and lay off the amount of the observed deviation on the dotted line which passes through it, or if no dotted line passes through it, then in a direction parallel to the dotted lines, to the right if the observed deviation be easterly, to the left if westerly, and mark the point so determined with a cross or dot in ink. Perform the same operation for each observed deviation, then with a pencil and a light hand draw a flowing curve, passing as nearly as possible through all the crosses or dots; and when satisfied with the curve in pencil, draw it in ink. The deviations thus laid off on the diagram can be readily applied to the ship's courses by the two following methods: 1st. From the compass course which has been steered to find the corresponding magnetic course to be laid off on the chart. Move in On the vertical line take the compass course. a direction parallel to the dotted lines till you arrive at the curve, and then move in a direction parallel to the plain lines till you get back to the vertical line. The point on the vertical line at which you arrive is the magnetic course required. EXAMPLE. With the deviations as in Table A, laid off on Diagram A. Assuming the courses steered by the standard compass to have been North, N.W., N.N.E., the corresponding correct magnetic courses to be laid off on the chart would then be respectively, N. 2° E., N. 67° 30' W., N. 41° E. Again, With the deviations as in Table B, laid off on Diagram B. Assuming the courses steered by the standard compass to have been N.E. by E., W. by N., N.N.W., the corresponding correct magnetic courses to be laid off on the chart would then be respectively, N. 40° E., N. 54° W., N. 41° W. 2nd. From the magnetic course as laid off on the chart, to find the corresponding compass course on which to keep the ship's head. On the vertical line take the given magnetic course. Move in a direction parallel to the plain lines till you arrive at the curve, and then move in a direction parallel to the dotted lines till you get back to the vertical line. The point on the vertical line at which you arrive is the compass course required. EXAMPLE. With the deviations as in Table A, laid off on Diagram A. Assuming the correct magnetic courses laid off on the chart to be East, S.E., West, the corresponding courses to steer by the standard compass would then be respectively, N. 62° E., S. 61° 30′ E., N. 63° W. Again, With the deviations as in Table B, laid off on Diagram B. Assuming the correct magnetic courses laid off on the chart to be E. by S., S.S.W., N.E., the corresponding courses to steer by the standard compass would then be respectively, S. 56° E., S. 19° 30' W., N. 62° 45′ E. The diagram can also be used for finding the correct magnetic bearings of distant objects as follows: Place one leg of the dividers on the vertical line at the position of the ship's head by the standard compass, and |