There are various graphic methods of delineating the deviation of the compass, and for converting correct magnetic courses into compass courses, and compass courses into correct magnetic courses ; that introduced here is due to Mr. J. R. Napier, F.R.S., of Glasgow. It is equally applicable whether the points on which observations have been made are or are not precisely equidistant; and only requires a moderate degree of dexterity in drawing a curved line. Construction of the Diagram.-As originally proposed the diagram consisted of a vertical line divided into 32 equal parts, representing the 32 points of the compass; it was also subdivided into 360 equal parts, representing degrees, reckoned from oo at North and South to goo at East and West; thus it might be considered as the margin of a compass card, cut at the North point, and straightened out. The diagram that accompanies this work is what has just been described, with the exception that the vertical line is lengthened by a point of the compass at top and bottom to give greater facility in drawing the curve of deviation, should an observation be near, but not exactly at North. The vertical line is intersected at each point of the compass by two straight lines—one plain, the other dotted-each inclined to the vertical line at an angle of 60°. Observations for the deviations should be made on as many points as convenient, but those on eight points (at or near N., N.E., E., S.E., S., 5.W., W., and N.W.) will be ample to furnish a curve sufficiently exact for practical purposes. Construction of the Curve of Deviation.--Easterly deviations are to be laid lown on the dotted line to the right of the vertical line ; Westerly deviations on the dotted line to the left. The method of constructing the curve will be best understood by explainng the projection corresponding to the observations No. 1 (thick curve) below:-No. I (THICK CURVE). No. 2 (THIN CURVE). NW. 26 40 W. N.W. 13 20 E. 1. Take (with dividers) from the vertical line a distance equal to deviation 64°, and lay it off on the dotted line, from North towards the left-deviation being W.; at the extremity of the distance make a dot or cross. 2. Take from the vertical line, a distance equal to 171°, and lay it off on the dotted line, from N.E. towards the right-deviation being E.; make a dot. 3. Take from the vertical line, a distance equal to 25°, and lay it off on the dotted line, from East towards the right-deviation being E. : make a dot. 4. Take from the vertical line, a distance equal to 171°, and lay it off on the dotted line, from S.E. towards the right-deviation being E.; make a dot. 5. Take from the vertical line, a distance equal to 61°, and lay it off on the dotted line, from South towards the right---deviation being E.; make a dot 6. Take from the vertical line, a distance equal to 8%, and lay it off on the dotted line, from S.W. towards the left-deviation being W.; make a dot. 7. Take from the vertical line, a distance equal to 25°, and lay it off on the dotted line, from West towards the left--deviation being W.; make a dot. 8. Take from the vertical line, a distance equal to 262°, and lay it off on the dotted line, from N.W. towards the left-deviation being W.; make a dot. 9. Repeat at the lower end of the vertical line the first admeasurement from North. 10. With a pencil and a light hand draw a flowing curve, passing through all the dots; when satisfied that the curve is good, drawit in ink. You then have a curve of deviations, by means of which the deviation on any compass course or any magnetic course may be found, as well as the compass course corresponding to any correct magnetic course. In a similar manner lay down the deviations corresponding to observations No. 2 (thin curve). Application of the Curve of Deviation : 1. To find the Deviation on any Compass Course. On the vertical (central) line find the given course; measure (with dividers) the distance from that point to where the curve cuts the dotted line proceeding from the point; that distance measured on any part of the vertical line will give the deviation in degrees. Thus, the deviation, by thick curve, on N.E. by N. is 13° E.; and on W.S.W.is 171° W. To find the Deviation on any Magnetic Course.—On the vertical line find the given magnetic course; measure with a pair of dividers along, or parallel to the plain line, until you reach the curve; the distance in the dividers measured on any part of the graduated central line will be the deviation on that magnetic course. II. From a given Compass Course to find the corresponding Correct Magnetic Course.-On the vertical line find the given compass course, from which move in a direction parallel to the dotted line till you arrive at the curve, and thence move in a direction parallel to the plain line till you get back to the vertical line. The point on the vertical line at which you arrive is the correct magnetic course required. (a) Thus, by thick curve, compass course N.E. by N. gives correct magnetic course N. 47° E., and compass course W.S.W. gives correct magnetic course S. 50° W. III. Given a Correct Magnetic Course to find the corresponding Compass Course.On the vertical line find the given correct magnetic course, from which move in a direction parallel to the plain line till you arrive at the curve, and thence move in a direction parallel to the dotted line till you get back to the vertical line. The point on the vertical line at which you arrive is the compass course required.(6) Thus, by thick curve, for correct magnetic course S.E., steer by compass S. 67° E.; and for correct magnetic course W. by N., steer by compass N. 51° W IV. Bearings require correction for the direction of the ship's head.--Thus, if the bearing by compass is N.E., and the ship is heading by compass S.E. on which the deviation is 28° W. (by thin curve), the correct magnetic bearing will be 28° to left of N.E., i.e., N. 17° E. (a) From compass course magnetic course to gain Depart by dotted and return by plain. Depart by plain and return by dotted.” N.E. S.W. Ans. N. 86° E. N. 19° W. N. 58° E. S. 29° W. Supposing you have steered the following courses by the Standard Compass, find the correct magnetic courses made from the above deviation table—(using thin curve). S.S.W. S.E. by E. E.N.E. W. by N. Ans. S. 33° W. S. 87° E. N. 50° E. N. 56° W. You have taken the following bearings of two distant objects by your Standard Compass as above; with the ship's head at S. by E., find the bearings, correct magnetic—(using thin curve). Bearings by compass S.W. } W. and N.W. by N. give Ans. Correct magnetic bearings S. 37° W. and N. 47° W., since the deviation on S. by E. is 13° W. (From thick curve).-Give the courses you would steer by the Standard Compass to make the following courses, correct magnetic. E. by N. W. by S. S.E. by S. N.N.W. Ars. N. 57o E. N. 74°W. S. 53° E. N. 10° W. The course allotted is the magnetic Course from the chart. (From thick curve).-Supposing you have steered the following courses by the Standard Compass, find the correct magnetic courses made. E. by S. W. by S. E. by N. N.W. by N. Ans. S. 54° E. S. 62° W. S. 75o E. N. 55o W. (From thick curve).—You have taken the following bearings of two distant objects by your Standard Compass with the ship's head at E. by N.; find the bearings, correct magnetic ? Bearings by compass N.E. and S.E. give Ans. Correct magnetic bearings N. 70° E. and S. 20° E., since the deviation on E. by N. is 25° E. (From thin curve).--Give the courses you would steer by Standard Compass to make the following courses, correct magnetic. E. by N. S.S.E. W. by N. N.W. by N. Ans. S. 73° E. S. 11° E. S. 75o W. N. 47° W. (From thin curve).-Supposing you have steered the following courses by the Standard Compass, give the correct magnetic courses. S.W. by W. N.W. E.S.E. S.S.E. Ans. S. 80° W. N. 32° W. N. 83° E. S. 43° E. (From thin curve).—You have taken the following bearings of two distant objects by your Standard Compass, with the ship's head at S.E. by S.; find the bearings, correct magnetic. Bearings by compass N.E. by E. and N.W. 1 N. Ans. Correct magnetic bearings N. 31° E. and N. 64° W., since the deviation on S.E. by S. is 25° W. (From thick curve).--Give the courses you would steer by the Standard Compass to make the following courses, correct magnetic. E. by N. S.S.E. W. I N. N.W. by N. Ans. N. 57° E. S. 39° E. N. 57° W. N. 18° W. Supposing you have steered the following courses by the Standard Compass, give the correct magnetic courses made good-(using thick curve). S.W. by W N.W. N. S.E. by E.' N. by W. Ans. S. 43° W. N. 65° W. S. 37° E. N. 23° W. You have taken the following bearings of two distant objects by your N. 397° W. W. 21 W. Bearings Corr. Mag. N. 354 E. N. 60} W. = 21 N.B. Bearings must always be corrected for the deviation due to the direction of the ship's head at the time the bearings were taken; in this case the deviation on W. by S. 1S. must be used. THE DAY’S WORK A SHIP'S RECKONING is that account by which it can be known at any time where the ship is, and on what course or courses she must steer to gain her port. Shaping the Course.-Having taken the departure, the course must then be shaped, and from that moment the run by log must be noted. Having decided on the track, the true or magnetic course can be taken from the chart; if the latter, allowance must be made for the deviation due to the direction of the ship's head to obtain the compass course. If sailing in a known current there should be deduced and allowed a proper amount for it. Should the wind be ahead, the tack must be chosen upon which the greatest distance will be made good towards the port of destination. The Log-Book is a book ruled in order to contain the daily copies of the remarks written at intervals during the day of everything connected with the progress of the ship, on her voyage, and any occurrences worthy of notice. It is strictly a journal, each page being ruled for one day; with other important entries superadded, it is the official, and only authentic, record of the daily proceedings on board ship, and is divided as follows, under the headings of :H. K. Courses. Winds. Lee | | Dev. Remarks, Monday, April ioth. The column on the left contains the 24 Hours from the noon of one day to the noon of the next, divided into two portions of 12 hours each. In the second and third columns are the Knots and Fathoms (or Knots and Tenths) the ship is found to run per hour, set against the hours when the log was hove. N.B.The log is generally hove once in two hours, and the intervening rate inferred. The “ patent log "gives the miles run between the intervals of observing it. The fourth column contains the Courses the ship steers. The eighth column, for Remarks, gives :—The kind of wind and weather -as moderate, fresh, a gale, squally, strong gale, foggy, cloudy, etc.; the state of the sea-as smooth, moderate, high, or heavy, etc.; the alteration of the sails—as tacked ship, squared the yards, etc.; the business doing a board, as to the employment of the crew; when the ship was pumped, and the water in the well; the ships sighted or spoken; it in soundings, the depth of water and nature of the bottom, etc.; the Variation of the compass; and what other remarks the officer of the watch thinks it his duty to insert. In the Royal Navy the time is reckoned as on shore, and has been kept in civil time since 1805, by order of the Admiralty. In the Merchant Service it has been the custom to begin the day at noon, and presumably this continues to be the case in the majority of ships, and with most Masters. |