Paper VII. For Second Mate, 1. Multiply 605 by 09 by logarithms. 2. 3. Divide 176 by 1.46 by logarithms. In latitude 38°, the departure made was 166m.: required the difference of longitude by parallel sailing. 4. Sept. 23, 1868: long. 100° E.: observed meridian altitude O 57° 20′ 30:" observer S. of sun: index error 1' 20" to add: eye 21 feet required the latitude. Additional for Only Mate. 5. Determine by Mercator's sailing, the course and distance from Cape Horn, lat. 55° 59′S. : long. 67° 16′W.: to Cape of Good Hope, lat. 34° 22′S. : long. 18° 29′E. 6. Dec. 16, 1868 at 4h 35m 32 A.M. apparent time at ship: lat. 40° 4'S. long. 24° 37' W.: the sun's rising amplitude was observed to be S.E. E.: required the variation. 7. Dec. 16, 1868: required the A.M. and P.M. tides at Peterhead. 8. Dec. 24, 1868: P.M. at ship: lat. 30° 17'S. observed altitude 50° 10′ 15′′: index error + 24": eye 19° feet: time by chronometer 24d 8h 20m 1s, which had been found 5m 27s slow on mean time at Greenwich, July 13, at noon, and gaining 65 daily: required the longitude. Additional for Chief Mate, 9. Dec. 16, 1868 at 8h 3m A.M. mean time at ship: lat. 15°13′N.: long. 112° 30' W.: observed altitude 20° 16′ 20′′: index error -7'': eye 17 feet: sun's bearing by azimuth compass S. 66° 10′ E. : required the variation. 10. Dec. 12, 1868: P:M. at ship: lat. by account 62° 34'S. long. 24° 23′E.: observed altitude near the meridian 46° 40′ N. : index error + 31′′: eye 16 feet: time by chronometer 12a 0h 54m 53s, which had been found 49m 48s slow on apparent time at ship, but the ship had made 17' diff. long. to W., since the error for time had been determined: required the true longitude. Additional for Master Ordinary, 11. Correct the following courses for local attraction, by Table at page 62, E. S.-N.E.E.-N. by E. E.-N.N.W. 12, Dec. 26, 1868: long. 165°W.: observed meridian altitude of Canopus, (a Argus) 67° 30': star S. of observer: eye 18 feet. required the latitude. EXERCISE ON CURRENT SAILING. SUPPOSE that a ship bound from a to c, is influenced by a current, setting as directed by the arrow, what will be her course? It is first to be understood that the distance set, is indicated by the length of the arrow; if necessary to obtain the course, draw a compass, and notice the direction of AB as near as you can. Draw a line from A to c, which will be the direct bear ing, then draw . CB and AD of the same length, and parallel to the arrow. A to B will be the course steered, and Ac the distance run, to fetch c, draw CD to form the parallelogram. LOCAL ATTRACTION. DEVIATION OF THE COMPASS.* LOCAL ATTRACTION is a term used to denote the influence of iron in disturbing the direction of the magnetic needle, * For fuller and more complete information on this subject, with rules for the application of Deviation and Variation, see Mrs. Janet Taylor's Epitome, pages 59 and 235. whereby, according to the extent and position of that metal in respect to the compass, a greater or less amount of deviation from the magnetic meridian is the result. This derangement of the compass, some 30 years ago scarcely noticed, must have been the source of numberless accidents to vessels, often accompanied by a great sacrifice of human life; for since that period, the careful researches of scientific observers have proved, that it would be extremely hazardous to allow the ships of H.M. navy, steamers, or iron-built vessels to proceed to sea, unless the amount of errors arising from the local attraction has been previously determined; it may suffice to mention, in illustration, that in some screw steamers the deviation, with the ship's head South, has amounted to more than fifteen points. It must not be supposed that the compasses on board vessels built of wood, are wholly free from the effect of local attraction; under ordinary circumstances, with no undue proportion of iron in the ship, the deviation may not be so great as to produce any serious error on the courses made: but the case is materially altered when the cargo consists in part, or wholly, of iron,—or in fact, when any quantity of that metal is placed in the vicinity of the binnacle. The amount of deviation on a given point of the compass is by no means a constant quantity, but it differs in different vessels, each having a local attraction peculiarly its own; it also varies with change of position on the globe; in north magnetic latitude, the north pole is attracted, in south magnetic latitude, the south pole,-increasing or decreasing as the dip of the needle increases or decreases: nor is it by any means the same in all parts of the same ship, and it may even alter from circumstances connected with the vessel itself, or the needle, or even both. It is therefore necessary that masters should be able to ascertain, as well as know how to apply, the errors arising from the local magnetic disturbance : for this purpose two methods may be adopted,-the first being the most approved method. Method 1.-With Two Compasses.-The ship must be placed in such a position that she may be gradually swung, and the two compasses being compared together to note their agreement, let one of them be placed in the binnacle in its usual position, and the other taken on shore beyond the influence of the attractive force; adopt such means that a good bearing of each may be taken, and as the ship's head is brought to each point in succession of the compass on board, at that instant let observation be made, thus,-the person on shore must take the bearing of the compass in the binnacle, and the person on board must take the bearing of the compass on shore, proceeding in this manner through the 32 points; these bearings must be tabulated after the following method: Direction of Ship's Head. Bearing of Shore Bearing of Compass Shore Compass. : Difference of The difference between the bearings will be the amount of deviation due to the local attraction of the ship, and is named East, when the north point of the needle is drawn to the eastward or right hand,-West, when it is drawn to the westward or left hand; and must be applied to the ship's courses in the same manner as the variation of the compass. Method 2.- With One Compass.-Having determined the true bearing of a conveniently distant object, let the ship be carefully swung to each point of the compass, and on each occasion, let the bearing of the object be taken: the difference between the true and observed bearing, will be the error of the compass, to be named and applied as in the former case. |