CHAPTER XXIX. COMPENSATION OF THE COMPASS. Section One: Illustration of Compensation by Means of 361. The observations. The experiments with the SCORESBY have the advantage of exposing to view the location and nature of the material that is the seat of the disturbing forces, the deviations produced by these, and the means taken to neutralize them: the observations used in this chapter were made in 1898; they exemplify what, in substance, would be done on a ship. The same compass, No. 24025, was used throughout: it was a four-needle liquid compass of the type described in Part Second-identical with those used in the Navy for standard compasses. The following was the condition of the SCORESBY during the experiments Fig. 553. The vessel is pivoted at P, the axis of rotation being the prolongation of the compass-pivot as nearly as it could be attained; the wheel W rolls on a graduated brass circle screwed to the floor, and by means of the handle K the vessel may be swung; the disturbing magnetic material is colored red and purple, the COMPENSATING appliances, green; the compass is represented by a single red arrow at N and the compensating magnets in the starboard angle by a single bar H, but the compass was the four-needle one specified above and there were four cylindrical magnets at H.. For convenience of reference, the various stages of the procedure are numbered within parentheses in this article. (1.) Removed all iron and magnets from the ScORESBY and its vicinity to the corners of the room, so that only the natural field of the Earth (as it existed in the room) should affect the compass. (2.) With conditions of (1), swung the vessel on 16 points and observed the bearing of the "electric sun": on different points it varied between N. 33° W. and N. 34° 30′ W. and included the parallactic angle; to indicate the magnetic direction of each heading, radii to the brass circle on the floor were drawn with chalk-keel lines of the vessel while on each heading. The procedure thus far would correspond to determining the magnetic bearing of a distant object; it is likewise analogous to finding the bearing of the Sun from Azimuth Tables. In this case the sun was fixed. (3.) Removed Compass from binnacle, and with all else as in (1), placed a small Horizontal Needle in the exact site of the compass and observed ten sets of ten oscillations each, noting the time with a stop-watch marking quarter seconds. The needle was about three inches long, in a brass case covered with glass. Amplitude of swing about 20° at beginning. Mean (T) of the ten sets is given in col. (3), Table 94: it was 17.92 sec. (4.) Removed horizontal needle and substituted a Dip Circle, Fig. 168: magnetic conditions otherwise as in (1), observed the Dip (67° 30') and ten sets of oscillations, the plane of the moving needle being transverse to the meridian. Amplitude about 20° at beginning. Mean (T) of the ten sets of oscillations is given in col. (4), Table 94. (5.) Removed dip circle, and substituted Balanced Needle in plane of compass-card, and conditions being otherwise as in (1), noted point of bead on scale (1.7), col. (5), Table 94, when the needle was horizontal-see Fig. 552. (6.) Observations (3), (4), and (5) correspond to the determination of same quantities on shore in a spot free from magnetic disturbance. (7.) Replaced the balanced needle by compass No. 24025, thus restoring, the conditions of (1). 12 0.8 0.6 0.4 0.2 0.8 0.6 0.4 0.2 10 N (8.) FIG. 552. Now placed the disturbing material as follows on the vessel, Fig. 553: The ship's head being NE. magnetic both by compass and radius on floor, placed three thick soft-iron rods at A, to produce both semicircular deviation and heeling error: the compass now indicated a heading of N. 38° 30' E., thus showing the effect of the rods to be 6° 30′ easterly deviation. These rods were 42 inches long and inclined at an angle with the vertical; their nearest ends were 12 inches from pivot of the compass. (9.) Next put a hollow soft-iron sphere B, 9 inches in diameter, forward and below plane of compass-card, with the nearest part of surface 18 inches from pivot: its effect was to cause 1° 30' easterly (quadrantal) deviation, for the ship's head by compass was now N. 37° E. (10.) Then placed a large thick plate C of soft boiler iron in the horizontal plane of the needles to produce quadrantal deviation: its nearest end was 14 inches forward of the compass, which now indicated N. 32° E., showing the effect of the plate to be 5° easterly deviation. (11.) Finally, put six powerful bar-magnets D on spar deck, horizontally, forward and below plane of needles and inclined to the fore-and-aft line, with their south poles nearest the compass and at a distance of 32 inches from the pivot: these magnets were 19 inches long. The compass indicated a heading of N. 10° 30' E., thus showing their effect to be 21° 30' easterly deviation. During the procedure of (8) to (11), both inclusive, the ship had been kept blocked on NE. magnetic. The (red-colored) disturbing materials E and F were added subsequently for another purpose and will be described further on. (12.) The foregoing completed the combination of (purple-colored) disturbing material, and with it in position as shown in Fig. 553, but without any compensating appliances on the binnacle, and also without E and F, swung ship and observed the bearing of the electric sun on each of the 32 compass headings, resting three to four minutes on each point. Comparison of these bearings with those of the electric sun when no disturbing matter was on the vessels, gave the deviations-Table 93. All this procedure is clearly analogous to steaming in a circle or swinging at compass-buoys, and observing azimuths of the sun. (13.) The compass was now removed from the vessel; the disturbing material A, B, C, D was still in place just as when the ship was swung for Table 93; no compensating appliances (H, V, or S) were on the binnacle, neither was E nor F: and under these conditions placed the same horizontal needle employed in (3) where it had been used there, and observed the times of five sets of ten oscillations each, with the ship headed successively on the eight principal magnetic points, as shown by radii on the floor. The mean values (T, T., T., etc.) of these observations are |