I will explain the method of obtaining the Long. in, from the Long. left and Diff. Long., using the following examples : Long. in 176 W. 176 E. Understanding that Longitude is reckoned E. or W. from the meridian of Greenwich to 180°. (1) If you are in east long. and sail east, Long. left must be increased by Diff, Long. (2) If you are in east long. and sail west, Long. left must be decreased by Diff. Long. (3) If you are in east long. and sail west, and Diff. Long. exceeds Long. left, take the less from the greater, and the remainder will be the Long. in, west. (4) If you are in west long. and sail west, Long. left must be increased by Diff. Long.. (5) If you are in west long. and sail east, Long. left must be decreased by Diff Long. (6) If you are in west long. and sail east, and Diff. Long. exceeds Long. left, take the less from the greater, and the remainder will be Long. in, east. (7 and 8) If the sum of the Long. left and Diff. Long. exceeds 180°, take the sum from 360° for the Long. in, which will have a different name from the Long. left. The Rule stated briefly resolves itself into this :-For Long. in, if Long. left and Diff. Long. are of the same name, take their sum; if they are of different names take their difference, and in this case the Long. in will be of the same name as the greater; but note well 7 and 8. ON TAKING A DEPARTURE-CROSS BEARINGS. Bound on a voyage you take what is called a DEPARTURE from the last or most conspicuous well-known point of land visible. If only one such point shows, as in the case of the ship at b in Fig. 6, you take its bearing by compass, and estimate its distance, both of which you enter in the log-book. (See log-book form, p. 42.) N.B. 1.-If your compass has no deviation, your compass-bearing will be correct magnetic, and will require no correction but for variation. (See log-book form, p. 42.) N.B. 2.-If your compass has deviation, your compass-bearing will be affected by the deviation due to the direction of the ship's heading, which you will enter (see log-book form, p. 44); and it will require to be corrected for deviation and variation. N.B. 3.-On reckoning up your Day's Work, the bearing at Departure must be entered in the Traverse as a Course in the opposite direction, for this reason ;— take the Lizard as your point of Departure, the position of which is well known; but the ship must be several miles from the Lizard, which we will say bears N.E. by E. 14 miles from the ship; then you can easily understand that the course from the Lizard to the ship would be S.W. by W., and the distance 14 miles; it is to this departure course to which the corrections must be applied. the If two good and well known points are visible, you take what are called CROSS-BEARINGS by compass; correct them for the deviation (if any) due to ship's heading, and then plot them on the chart by the compass (magnetic) that you find near your position; for example, you are in a ship at a in Fig. 6; point with the lighthouse on it bears correct magnetic N. by W., and the point with the tree on it E. by N.; you lay the parallel rules over the compass on your chart at N. by W. and work them to the lighthouse, preserving the direction; you then draw the line from the lighthouse to a; similarly you lay the parallel rules over the compass on your chart at E. by N. and work them to the tree; you now draw the line from the tree to a; where the lines cut was the ship's position on the chart when the bearings were taken; and the distance of the ship from lighthouse or tree can be at once measured by taking in the dividers the distance between either and the ship, and referring the space between the two points of the dividers to the graduated meridian on the side of the chart. Let the angle of the bearings be as near as possible 90° or 8 points. When sailing along the land, cross-bearings will always determine your position from it, allowing the proper corrections on the compass. When working your Day's Work, day by day, your first Departure is, of course, the land you leave; for subsequent days you will take as your departure. the position (Lat. and Long. by observation if possible) of the previous noon. THE DAY'S WORK. We will now proceed to the solution of the Day's Work-which is the determination of the Ship's Position by Dead Reckoning, the basis of which is to be found on the Log-Board,—or in the Log-Book, which has been copied from the Log-Board. In order to do this effectively, it may not be amiss to give a brief summary of much that has already been stated at some length: but do not suppose that you will have a good knowledge of what the Day's Work means, and of the method of solving it by merely reading the summary, because you will not: I have not written a word more than you ought to be well up in, and have perfectly impressed on your mind, before you come to this part of Navigation. In going through these Day's Works verify every figure by reference to the Traverse Tables, &c.-take nothing as granted to be correct: your doing so, and your mere acquiesence in the solution, does not prove that you under stand it. When correcting a Course or Bearing, always suppose yourself standing in the centre of the Compass, looking in the direction of the Course or Bearing. A Compass Course may be affected by Leeway, Deviation, and Variation. A correct Magnetic Course can be affected by Leeway and Variation, nɔt by Deviation. A Compass Bearing may be affected by Deviation (due to the direction of the ships' heading or course), and by Variation. A correct Magnetic Bearing can be affected only by Variation. When correcting for Leeway Allow it to the right if on the port tack ; Allow it to the left if on the starboard tack; that is, in a direction from the wind. When correcting a compass course for Deviation- Allow it to the left, if Westerly. When correcting a compass course for Variation— Allow it to the left, if Westerly. N.B.-The application of the Deviation and Variation to the compass courses to get True Courses may be expressed in another way, remembering that + (plus) signifies additive and · (minus) substractive : Easterly Dev, and Var, are + to all Compass Courses between N. and E. S. and W. from all Compass Courses between N. and W. + to all Compass Courses between from all Compass Courses between S. and E. N. and E. 1. Reverse the departure bearing; express it in degrees, and apply the deviation (due to the direction of the ship's head), and the variation; the result is to be taken as a course. 2. Correct each course in succession, allowing the leeway (if any), applying the deviation (appertaining to each course), and also the variation. Note.-Allow the leeway in points before expressing the course in degrees. 3. The current being correct magnetic, treat it as a course, but only apply the variation. Note.-The drift is to be taken as a distance. 4. The true courses being thus found, are then, with their respective distances, to be entered in the following form: 5. Enter Traverse Table II. with the several courses and distances, and fill up the D. Lat. and Departure columns. Remember to read from the top of Trav. Tab. if course is less than 45°, but from the bottom if over 45°. Also, if Dist. exceeds 300 miles, enter the Trav. Table with half the Dist., take out the corresponding D. Lat. and Dep., and then double them. 6. Add together the quantities in the N. column, and also in the S. column; then, take the difference of the two sums, giving the remainder the name of the greater quantity (N. or S.): the result will be the Diff. Lat. made good. 7. Add together the quantities in the E. column, and also in the W. column; then, take the difference of the two sums, giving the remainder the name of the greater quantity (E. or W.); the result will be the Departure made. 8. For the Course and Distance made good, enter Trav. Tab. II. and seek out the page where the D. Lat. and Dep. made are side by side, reading headings of columns from the top if D. Lat. is greater than Dep., but from the bottom if D. Lat. is the less. The course (in degrees) is taken from the top of the page if D. Lat. is greater than Dep., but from the bottom if D. Lat. is the less. The Dist. comes from Dist. column alongside the given D. Lat. and Dep.: name the Course according to the D. Lat. and Dep., and write the result thus-Course made good N. or S. . . .° E. or W.; Dist. miles. 9. For the Lat. in.-Under the Lat. left write the D. Lat.; if both have the same name (N. or S.) take their sum for Lat. in, of the same name as Lat. left: if one is N. and the other S. take their difference for the Lat. in, of the same name as the greater. 10. For the Middle Lat.-Add together the Lat. left and Lat. in, if both have the same name (N. or S.), and divide the sum by 2; if one Lat. is N. and the other S., take their difference and divide the remainder by 2. 11. For the Diff. Long.-Enter Trav. Tab. II. with Mid. Lat. (as a course) and Dep. (in D. Lat. col) the Dist. (in Dist. col) is the Note.-If Dep. exceeds 300, take the half of it, and having found the Dist. corresponding thereto, double it. At the equator, or if the Lat. in is less than 5° or 6° the Dep. may be taken as the Diff. Long. 12. For the Long. in.—Under the Long. left write the Diff. Long. with the same name as the Dep. If Long. left and Diff. Long. have the same name (E. or W.) take their sum for the Long. in, of the same name as Long. left; except this sum exceeds 180°, in which case subtract it from 360°, and mark the remainder W. when Long. left is E., but E. when Long. left is W. If Long. left and Diff. Long. have different names (one E. and the other W.), take their difference for the Long. in, of the same name as the greater. DAY'S WORK No. I. The Courses, Leeway, and Variation being in points of the compass, you will retain them so, and use Trav. Tab. I. 1st. Begin with the Departure; reverse it, and enter it as a course after correcting for variation. 2nd. Take each course in succession, correct for leeway (if any), and then for variation: also sum up the distance run on each course. 3rd. Take the current as the last course, and correct for variation. 4. Then proceed as directed in the summary, p. 40-41; or as your knowledge of the earlier part of this work assists you, without reference to the summary. NOTE. This is a copy of the Log-board, divested of the bulk of the remarks. Lat. left. 49° 58′ N. Mid. Lat. 49 49 Mid. Lat. 49° and Dep. 91. 2 (in Lat. col.) in Trav. Tab. II. give 139 in Dist. col., for Diff. Long. Mid. Lat. 50° and Dep. 91.2 (in Lat. col.) in Trav. Tab. II. give 142 in Dist col., for Diff. Long. Take 141 for Diff. Long. corresponding to Mid. Lat. 49§°. Diff. Lat. 18 4 S. and Dep. 912 W. in Trav. Tab. II. give |
