55. Correct the compass course N.W.W. for deviation W. (from deviation table, p. 16), and also for variation or compass 31 W. Find the true course in each of the following examples, by correcting for deviation from table, p. 16, and for variation: To find the compass course, having given the true course and deviation. Easterly deviation allow to the left. NOTE. The true course should first be corrected for variation (if any) by Rule (h), (which is similar to the above), so as to get a compass course nearly, and then this course for deviation, from table, p. 16. EXAMPLES. 62. Required the compass course, the true course being W.S.W.W., variation O, and deviation W. (see table.) 63. Required the compass course, the true course being S.W., variation of compass 24E., and deviation as in table, P. 16. Required the compass course in each of the following examples (for deviation, see table, p. 16): This correction is the angle which the ship's track makes with the direction of a fore and aft line: it arises from the action of the wind on the sails, &c. not only impelling the ship forwards, but pressing against it sideways, so as to cause the actual course made to be to leeward of the apparent course, as shown by the fore and aft line. The amount of leeway differs in different ships, depending on their con struction, on the sails set, the velocity forwards, and other circumstances. Experience and observation, therefore, usually determine the amount of leeway to be allowed. Rule (1). The method of correcting for leeway will be best seen by the following example: Suppose the apparent course is S.S.W. W., and leeway 2 points, the wind being S.E., required the correct course. Draw two lines at right angles to each other towards the cardinal points of compass, and a line, as ca, to represent (roughly) the course of the ship, and another to represent the direction of the wind (as the arrow in fig.); then it will be seen that the corrected C course, as c T, will be to the right of the apparent course; the observer being always supposed to be at the centre, c, and looking towards the cardinal point from whence the course is measured; hence Correct the following courses for leeway, so as to find the Correct the following compass courses for deviation, varia tion, and leeway, so as to find the true courses. The deviation is found in table, p. 16, and the variation of compass is supposed to be in each example 2 W. To find the course and distance from one place to another, having given the latitudes and longitudes of the two places. (1.) Find true difference of latitude, meridional difference of latitude, and difference of longitude: reduce the true difference of latitude and difference of longitude to minutes, attaching thereto the proper letters. Rules (a), (b), (d). (2.) To find the course. From the log difference of longitude (increased by 10), subtract the log. mer. diff. latitude; the remainder is the log. tan. course, which find in the tables, and place before it the letter of the true difference latitude, and after it the letter of the difference longitude, to indicate the direction of course. At the same opening of the tables, take out the log. secant course. (3.) To find distance. Add together log. secant course, and log. true difference latitude; the sum (rejecting 10 in the index) will be the log. distance, which find in the tables. EXAMPLES. 78. Required the course and distance from A to B. Required also the compass course in the above example: var. of compass being 2 points W, and deviation on account of local attraction, as in table (p. 16). See Rule (k). attaching to each logarithm taken out its name or designation, as in the following example: * Degrees are converted into points, or the converse, by means of the table for that purpose in the nautical tables. |