B = sin B [1 + sin D + 1⁄2 versin B- versin C] + sin C sin E = +.214 [1 +.0250+.00193.00098]+.0443 .00378 =+.2197 90. Determination of λ.-This coefficient is the one which expresses the proportion of the mean horizontal force northward of the earth and ship to the horizontal force on shore and may be found from equation (27) written as follows, when the horizontal force and the deviation for the magnetic azimuth z are known in addition to the exact coefficient V, C, D, and E. H' In the above equation I' the horizontal force of earth and ship combined; Hthe horizontal force of earth, considered as unity; = T being the time of n vibrations (say 10) of a small horizontal needle, 3 to 4 inches long, on shore in a place free from local magnetic disturbances; T" that of the same number of vibrations of the same needle, the center of which is in the same place exactly as that occupied by the center of the compass needle, when the compass is in place. Great care should be exercised in taking the vibrations, and the mean of a number of determinations should be used, since the error of a single set might be comparatively large. The equation H' T2 = H T2 is true only for infinitesimal arcs of vibration, but may be taken as sufficiently exact for all practical purposes if the arcs do not exceed 20°. However, the amplitude of arc should be as small as possible consistent with obtaining 10 well-defined vibrations. The place on shore where the needle is vibrated should be free from local attraction, a fact that may be determined in the following way, namely: place a compass on its tripod and set up a staff about 50 yards distant, note the bearing of the staff per compass; interchange tripod and staff and again note the bearing of staff. Do the same thing on a line perpendicular to the first line. If the bearing and reverse bearing in each case differ by 180°, the locality may be assumed free from magnetic local influences. The horizontal force instrument.-This instrument is used in finding the ratio of the horizontal force on board ship in the position of the compass to that on shore. It consists of a cylindrical brass case, with a removable glass cover, mounted upon a rectangular base which is provided with levels and leveling screws. The case contains a horizontal circle graduated to degrees, and in the center a pivot which supports a small lozengeshaped magnetic needle fitted with an adjustable sliding weight to counteract the dip and capable of vibrating freely in the horizontal plane. Observations for horizontal force ashore.-Find a level spot free from local attraction, level the horizontal force instrument and orientate it. By means of a small magnet draw the needle aside about 20°, quickly removing the magnet to a proper distance. Then as the needle passes the zero line the first time "mark the time" or start the stop watch, as the needle passes the zero line the second time count "one,” at the next passage "two," and so on till the count "ten," when the time is again noted or the stop watch stopped. The interval of time will be the time required by the needle to make 10 vibrations. Observations on board.-Observations are similarly taken on board, the center of the horizontal force needle occupying the exact place usually occupied by the center of the compass, which with all correctors must be removed to a safe distance. The horizontal force instrument is leveled on a brass table in the compass chamber, the spindle of the table entering the central vertical tube of the binnacle. The magnetic azimuth and deviation may be determined by any one of the usual methods. The coefficients should have been determined as accurately as possible on equidistant compass courses; 24, 12, or 8 equidistant headings. If the observations be taken on four equidistant magnetic azimuths we will have because, the summation of the sines and cosines being zero, the exact coefficients will disappear. NOTE: In the service compass, the plane of the needles is threefourths of an inch below the bottom of the wyes in which the compass rests when placed in the binnacle, and it may be located by placing in the wyes a straight edge at the center of which is pasted a piece of paper projecting vertically downward of an inch. == Ex. 9.-Given the exact coefficients of a "Monitor's Standard" A = .0092, V = + .2197, C = − .0863, |
