All errors of the compass may be classed as variation, deviation, or local attraction. 42. Variation is the angle included between a magnetic and terrestrial meridian. This error arises from the magnetic poles not coinciding with the terrestrial ones, and is due entirely to the influence of the earth on magnetic needles. 43. Deviation is the angle included between the magnetic meridian and a vertical plane passing through the poles of the compass needle; or it is the error caused by iron in a ship's build or her cargo. It is produced by some disturbing cause connected with the ship herself. Hence compasses should be placed as far as possible out of the influence of individual masses of iron: therefore positions near the iron rudder-head, the iron axle or upright of the steering wheel, near the foot of an iron mast, near the iron funnel in steamers, &c., should all be avoided. 44. Local Attraction is the error caused by some disturbing force unconnected with the ship or cargo, but belonging entirely to the locality, such as iron cranes, mooring posts or chains, water-pipes around a dock, another vessel near, or volcanic or other magnetic bodies like those found in Elba, St. Helena, and the Falkland Isles. Deviation may be distinguished from the other two errors by its dependence both for amount and direction on the position of the ship's head, and the angle at which the ship is heeled, whilst the other two are independent of the ship's action. A change of deviation may be expected— (a) With a change of magnetic latitude. With a change in direction of the ship's (d) With the heeling of the vessel. (e) With a heavy blow, as being struck by a sea. With the appearance of the aurora. (g) With the ship (if built of wood) being struck by lightning. Those marked a, c, d, e are the causes of change most frequently forced on the attention of the mariner, and no opportunity of testing the accuracy of the compasses should be neglected by him. 45. 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 every point of the compass, this is called swinging her; and care must be taken before a ship is swung to see that she is beyond all magnetic influence from surrounding objects, and that the vessel herself is equipped ready for sea. 46. (a) By reciprocal Bearings.-A compass is first compared with the standard compass whose deviation is required to see what error may arise from a difference of centering, mounting the card on the needle, or other causes. The standard compass is then put in its place and the other compass set up on shore, where it will be free from all disturbing causes, and can be conveniently seen from the standard. The ship is then swung, and at a given signal the position of the ship's head is observed as well as the bearing of the shore compass from the standard, and at the same instant the bearing of the standard compass is taken from the shore compass, and arranged in a table as follows:: A column may be added to insert the time of observation, to serve as a check in case a discrepancy should arise. It is evident that the bearings by the two compasses would be exactly opposite, if nothing but the directive force of the earth influenced either: but, as the vessel disturbs the one on board, that one must have deviation which is found by reversing the shore bearings, and taking the difference between them so reversed, and the bearings as taken by the standard. This is registered in the table opposite the ship's head, and is marked E. or W. according as the reversed shore bearings are to the right or left of the others. The 47. (b) By taking the Bearings of a distant Object. -A prominent object must be selected whose distance must be very great compared with the diameter of the circle described by the standard compass in swinging the ship; generally from six to eight miles will be found sufficient. bearing of the selected object must be taken and registered as the ship's head is brought to every point of the compass; and its correct magnetic bearing is the mean of the whole so registered; but in practice it is found sufficiently accurate to take the mean from the eight principal points. The differences between the correct magnetic bearing thus obtained and the compass bearings on every point is the table of deviations required. The form used is as follows, and the correct magnetic bearing is known to be N. 63° W. The mean of the bearings on the eight principal points is N. 62° 52′ W., and this, therefore, is assumed to be the corect magnetic bearing of the object. By using this value there will be a difference only of 7' from the true with which this table is calculated. 48. (c) By Marks on the Dock Walls.—This is a very convenient method where it can be practised. At Liverpool and Cronstadt there are marks painted on the dock walls showing the magnetic bearings of some conspicuous object inland. As a ship is swung the difference between the correct magnetic bearing and that shown by the standard compass is seen at once, and can be registered without trouble. 49. (d) By Azimuth and Amplitude Observations. -The compass errors obtained by observation of celestial objects on board ship always contain variation and deviation, and may also contain local attraction, if the ship be near any disturbing cause. By working an amplitude or azimuth, the true bearing is always found, the difference between which and the bearing by the standard compass at the time the sight was taken gives the whole error of the compass for the particular position the ship's head was in at the time. Declination or variation charts will always give the amount of error due to that cause; and the difference between the variation and the compass error is the deviation, easterly when the compass error is to the right, and westerly when it is to the left of the variation, reckoning from the north. By continuing this process to the eight principal points of the compass, a complete table can be formed by Napier's Graphic Method, to be explained hereafter. Example 1.-Suppose the compass error obtained by an amplitude on board ship to be 17° 30' W., and the variation from the chart for the ship's position to be 3o E., what is the deviation? Compass error 17° 30' W. Variation Difference=Deviation 3 0 E. Here it must be understood that the difference is the algebraic difference which may be thus expressed; when of contrary names add, but when of the same name subtract, to get the difference. 50. How a Deviation Table is used.--When the deviation is ascertained, it is applied precisely the same as variation; that is, in correcting a course |