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August, 1920 of the sun by compass, which bearing ss taken as the supplement of the com- pass azimuth. That bearing is. conse- quently subtracted from 180 degrees. It would appear then to be compara- tively simple to figure the compass er- ror from these two known angles, but practical navigators sometimes find this confusing. Ai simple' method. (is: to plot the two angles out as in Fig. 9, The angle SOC is the compass azimuth and the angle SOT is the true azimuth. The true is to the left of the com- pass, therefore, 'the compass error is westerly. Having found the total compass er- ror, it is next mecessary to find the deviation. If it is a magnetic compass we already know the variation and it must be remembered that this variation oO FIG. 11--DEVIATION EAST is included in. this total error. It is advisable here again to plot out the two angles as in Figs. 10, 11 and 12. In the first of these diagrams, the angle NOV is the angle of variation which is. easterly, and the angle NOE is. the angle of total compass error which is westerly, It is obvious here that the . deviation is the combination of these two or the angle /OE. In Fig. 11, the total error and the variation are designated similarly, but since both are easterly, the deviation is the differ- ence between the two or the angle VOE. Another application of this prin- ciple is iliustrated in Fig. 12 and this Principle may be applied in figuring the direction of the deviation for any combination of circumstances. To illustrate practically this principle of finding the compass deviation and error, assume that on April 16, 1919, the ship was in latitude 37 degrees, 18 ae S and longitude 93 degrees minutes W. The compass bear- ay ENE. The local mean time . eee and 43 minutes, a.m. The Setved altitude is 23 degrees 24 min- THE MARINE REVIEW utes, the height of the eye being 18 feet. The compass variation is known to be 15 degrees 45 minutes E and the ship is headed south. We wish to find the true course and the deviation. The method is illustrated in Table VII. There are other ways of figuring out the same result. For instance by sub- tracting the variation from the com- pass bearing we find the magnetic bear- ing, which subtracted from 180 de- grees will give the magnetic azimuth. Having the true azimuth and the mag- netic azimuth, we have but to subtract the one from the other to get the deviation. It would be: wise, however, to plot these two angles to make certain that the direction of the deviation is correctly named. By this method it would work out as follows: | Coe Bi 67 deg. 30 min. Var; 15 deg. 45 min. E MiB en 08 dee ie 180 deg. M. Z--S 96 deg. 45 min. E T. Z--S 124 deg. 34 min. E Dev. 27 deg. 49 min. W Again we might apply the variation to the true azimuth which would give the true magnetic bearing. Applying this last to the compass azimuth we obtain. the deviation. In this: case it would work out as follows: TT, ZS 124: dee. 34 min. & Var.-- 15 deg. 45 min. Ey 469 FIG. 12--DEVIATION WEST T. M. B.--140 deg. 19 min. C. Z--..< 112, dég.; 30 man. Dev. 27 deg. 49 min. W In this example the ship has been heading south (180 degrees) and the compass error was west (12 degrees 4 minutes), so that the course by new compass would be 167 degrees 4 min- utes, or by old compass S 12 degrees 4 minutes E. (To be continued) The 9400-ton steel ship Srnasta, being completed at San Francisco, has been assigned to the Columbia-Pacific Ship-. ping Co. for operation in tramp service. Local mean time 16d oe 43m 00s a.m. (If Local mean time 15d 20h 43m 00s Longitude in time Greenwich mean time 16d 2h 56m 28s Correction 2 min. 40.5 sec. 9 deg. 53 min. 17 sec. N 90 deg. 99 deg. 53 min. 17sec. S 23 deg. 24 min. True Declination Polar Distance 2) 160 deg. 45 min. 2 Sum Polar distance 99 deg. 53 min. Remainder 180 deg. : 55 deg. 26 min. $124 deg. 34 min. E True Azimuth (T.Z.) 'muth (T.Z.) $124 deg. 34 min. E rR (C.Z.) §112 deg. 30 min. E Table vil Method of Determining Compass Variation a.m. we must substract a day and add 12 hours) 6h 138m 28s W--93 deg. 22 min. W Declination for Apr. 16-- 9 deg. 50 min. 36.4 sec. hourly variation 00 sec. (No. Indicated error) height of eye 9 min. 43 sec. 23 deg. 383 min. 43 sec. ....secant.....0:03782 Latitude 37 deg. 18 min. 00sec. ....secant.....0.09937 Polar distance 99 deg. 53 min. 17 sec. 00 sec. 80 deg. 22 min. 30sec. . 19 deg. 30 min. 47 sec. (Bowditch, Table 45) Compass Bearing ENE--67 deg. 30 min. 12 deg. 04 min. W --Compass Error | 15 a 45 min. E --Compass Variation 27 deg. 49 min. W --Compass Variation (See Figs. 10, 11 and 12.) it were p.m. we would let this stand but since it is 53.50 x 8 160.5--2 min. 40.5\sec. j, cosine.... .9s22361 17 sec. Ge COsINe 3.7. - 9.97430 * 19.33510fLogarithmic haversine of atural, haversine.21632 0.21632/is Natural haversine of 5 deg. 26 min. 180 deg. ' : 67 deg. 30 min. $112 deg. 30 min. E. Compass Azimuth (C.Z.)