them down in the columns headed with the same letters as in course. Previously to opening the traverse table, fill up the columns of true difference latitude and departure not wanted, by drawing horizontal lines; this will frequently prevent mistakes. (3.) If the ship does not sail from a place whose latitude and longitude are known, her bearing and distance from some near object, as a church-spire, &c., must be ascertained, and also its latitude and longitude. Then the ship is supposed to sail from this known object to her anchorage, her course being the opposite to the bearing of the object from the ship. This course must be corrected like the rest for variation and deviation, and inserted in the table as an actual course, with the distance of the object as a distance. (4.) If a current sets the ship in any ascertained direction, and with a known velocity, these also may be conceived to be an independent course and distance, and must be corrected for variation, and should be for deviation also, if the latter correction is appreciable, which is rarely the case. (5.) To find the latitude in. The quantities in the four columns of true difference latitude and departure being added up separately, the difference between the north difference of latitude and south difference of latitude, with the name of the greater, will give the true difference of latitude, made at the end of the day. The departure is found in a similar manner. Apply true. difference latitude to latitude from, so as to obtain the latitude in. (6.) To find the longitude in. Add together log. sec. mid. lat. and log. departure, the result (rejecting 10 in the index) is the log. difference longitude. Find this in table, and thus the longitude in is found.* The following example, worked out in detail, will perhaps * Or thus:-To find diff. long., add together log. M.D. lat. and log. dep., and from the sum subtract log. T. D. lat. ; the remainder is the log. diff. long., which find in the tables. be sufficient to explain the operations directed in the above general rule. EXAMPLE. 112. April 27th, 1852, at noon. A point of land in latitude 36° 30' S. and longitude 110° 20' W. bore by compass E.N.E.E. (ship's head being S. E. by S. by compass), distant 14 miles; afterwards sailed as by the following log account; required the latitude and longitude in, on April 28th, at noon. (1.) The column in the above table headed deviation should be formed from the general table of deviations (p. 16) previously to correcting courses. Thus, in the first course in the preceding table, the ship's head is S.W.W.; looking in the deviation table we see that the corresponding correction is W. or 1. (Rule i.) (2.) Form a table such as below, by writing down the headings, points, courses, &c., over the seven columns which are to be filled in with the corrected courses, &c. be made mentally, and the diagram dispensed with. To correct the departure course which is W.S.W.W. (the opposite bearing to E.N.E.E.). Draw a line roughly in the fig. W.S.W. W. as Cl; it is or 7 pts. 3 qrs. left of N., or W. N. dist. 14'. Insert this course and distance in table below. First Course.-S.W.W. Draw a line in fig. S.W.W. as C2; then The distance 8.2' is found by adding up the hourly distances until the course is altered, at 4 o'clock. Insert this course and distance in the table. |