Example 6.-What is the deviation and course to steer by compass in order to make good a magnetic course of N. 80° E.? Mag. course N. 80° E. Lesser mag. co. N. 69 E. diff. 11° Lesser mag. co. N. 69° E. Greater mag. co. N. 82 E. diff. 13° Dev. 19° E. Dev. 22 E. diff. 3° Now, if 13° give a change of 3°, what will 11° give? N.B. To make good a magnetic course from chart if a vessel is making leeway, the leeway should be eliminated by applying it to the magnetic course before finding the deviation by allowing the leeway to windward of the magnetic course taken from the chart. Example.-Mag. course N. 20° E., ship making 20° of leeway, wind N.W. Apply the leeway to windward, that is, the mag. course will be North; now find the deviation for that mag. course. CLOSING REMARKS ON CHART WORK It must be obvious to the thoughtful navigator that some localities are easier to navigate than others. Amongst the most difficult must be placed the seas bordering the coasts of the British Isles and Northern Europe, because the waters in these seas are ever swinging to and fro under the influence of strong tidal streams of varying direction and velocity, rendering navigation a difficult and far from exact science. Whenever a course is to be set in these seas reference should always be made to the "Atlas of Tidal Streams," which shows the probable set of the tidal stream in all localities for one to six hours before and after high water at Dover, thus enabling the navigator to make a due allowance on the course for the state of the tide at the time the course is being set, and also what changes may be expected as the vessel proceeds on her way. It follows, then, from what has been said, that too much care cannot be taken in setting courses, and when a course has been carefully and judiciously set from a known position to any other fixed position it should be rigorously followed until the distance between the two points has been made good; but no opportunity of finding the ship's position should be lost, and a fresh course set, when found necessary. If a vessel, from any cause, be stopped when amongst strong tidal. streams, allowance should be made for the set and drift in the interval, and should the water be shallow the deep-sea lead should be dropped over the side and the line allowed to run out; the direction of the line would, if there were no wind, show the direction of the set. When sailing on a course which has been set to counteract the effect of a current it must be distinctly understood that the ship is supposed to be always on the straight line joining the point of departure with the point of destination, but with her head on the course that counteracts the effect of the current. In low-lying localities with no prominent points suitable for getting a good fix recourse must be had to the use of the lead in order to avoid getting into too shoal water. In shallow water and strong currents such as are experienced outside Rangoon and the entrance to the River Plate the ground log is the only reliable speed indicator and should always be used. In rounding points never adopt the "rule of thumb" method of conning the ship so as to keep the point abeam as she comes round, because distance cannot be correctly estimated by the eye which never takes into account the effect of refraction, which varies in its effect according to the state of the atmosphere. The "Vertical Angle" should be used in rounding points to guard against getting into danger; and when it is impracticable to use the Vertical Angle," owing to the object being too far inland, the horizontal angle should be used, as the distance of the objects inland in no way vitiates the excellent results obtained by this method. " In foggy weather it is well to remember that the distinctness or faintness of sound cannot be relied on as a guide to distance off, but the direction from whence it comes, coupled with the depth of water and the nature of the bottom obtained by means of the lead, would, in some localities, give a fair fix. It is a wise practice to take a few casts of the lead, in fine weather when the ship's position is known, as the results obtained might give confidence when the ship's position is uncertain. Never pass headlands, lighthouses, or light-vessels without getting a good fix by one of the foregoing methods. 1 A substance containing lime. 2. A group of very minute organisms having a hard flinty outer skin. 3. Many-celled organisms. 4. Deep sea foraminifera. 5. A certain branching form of coral. 6. Black oxide of a certain metal. 7. A mixture of lime and clay. 8. A class of mollusc with small wing-like appendages. 9. Ooze containing small shells. 10. Volcanic ashes. 11. Soft sandy stone. The position of the buoy or beacon is the Centre of the Base, and is usually indicated by a small circle. ↑ Alt. (Alternating) signifies a Light which alters in colour. * The number in brackets, after the description of Group Flashing or Group Occulting Lights, denotes the number of flashes or eclipses in each group. ↑ Occasional Fog Signal means a signal which is only given in answer to vessels' signals. The height given against a light is the height of the focal plane of the light above high water. Ordinary spring tides, or above the sea level in cases where there is no tide. The visibility of lights is given in nautical miles, assuming the eye of the observer to be 15 feet above the sea. Bearings of lights are given from seaward. TIDES H.W.F. & C. IXh. 25m...........high water full and change. The hours are expressed in Roman figures, except 2h. *H.W. or L.W. always refers to high water or low water of ordinary spring tides, unless otherwise stated. The period of the tide, at which the streams are running in the direction of the arrows, is denoted as follows The rise of tide is measured from mean low water of ordinary spring tides, unless otherwise stated. |