said to be 4 points to the right of N; E.b.N. is 7 points to the right of north. Each point, moreover, is subdivided into quarter points, and named from the adjacent points; thus, 2 points to the right of north is N.N.E. E.; 7 points to the right of north is E.b. N. E. or E.IN. The other three quadrants are divided and referred to in a similar manner. Attached to the compass, and coinciding with the line N. S., is a magnetic bar of steel, by means of which the card, when balanced on a fine point near its centre, will indicate the compass bearing or direction of any object beyond it. Thus, the compass being placed near the helm, the bearing of the ship's head is seen at once, and the direction in which the ship is steered is readily noted. The Log-line. The log is a flat piece of thin wood of a quadrantal form, loaded in the circular side with lead sufficient to make it swim upright in the water; to this is fastened a line about 150 fathoms long, called the log-line, which is divided into certain spaces called knots; the length of each knot is supposed to be the same part of a nautical mile (6120 feet) that 6120 half a minute is of an hour, hence 1 knot = = 51 feet. so on. 120 If, therefore, 1 knot runs out in half a minute (shown by a half-minute glass), the rate of the ship is supposed to be 1 mile an hour; if 2 knots, the rate is 2 miles an hour, and The length of the knot is very rarely so much as 51 feet, and the hour-glass used is not always a half-minute glass; various modifications of the two instruments are made, to render this method of measuring the ship's way tolerably correct; these will be more clearly seen in the use of the instruments themselves. Correcting Courses. Three corrections are sometimes necessary to be applied to the course steered by compass, to reduce it to the true course; and the converse. These are called: (1.) The variation of the compass. (2.) The deviation of the compass. (1.) The Variation of the Compass. The magnetic needle seldom points to the true north. Its deflection to the east or west of the true north is called the variation of the compass; it is different in different places, and it is also subject to a slow change in the same place. The variation of the compass is ascertained at sea by observing the magnetic bearing of the sun when in the horizon, or at a given altitude above it. From this observation the true bearing is found by rules given in nautical astronomy. The difference between the true bearing and the observed bearing by compass determines this correction. The method of correcting the course for variation will be more readily understood by means of an example. Suppose the variation of the compass is found to be 2 points to the east, that is, the needle is directed 2 points to the right of the north point of the heavens; then the N.N.W. point of the compass card will evidently point to the true north, and every other point on the card will be shifted round 2 points. If, therefore, a ship is sailing by compass N.N.W., or, as it is expressed, the compass course is N.N.W., her true course will be north; that is, 2 points to the right of the compass course. In a similar manner it may be shown that, when the variation is 2 points westerly, the true course will be 2 points to the left of compass course. Hence this rule : Rule (g). To find the true course, the compass course being given. Easterly variation allow to the right. From the preceding considerations it will be easy to deduce the converse rule, namely: Rule (h). To find the compass course, the true course being given. Easterly variation allow to the left. 42. Find the true course, having given the compass course N.W.W. and variation 34 west. 43. Find the compass course, having given the true course W. N. and variation 34 W. Find the true course in each of the following examples: * When names are alike, (that is, both left or both right,) add: when unlike, subtract, marking remainder with the name of the greater. (2.) Deviation of the Compass. This correction of the compass arises from the effect of the iron on board ship on the magnetic needle, in deflecting it to the right or left of the magnetic meridian. The increased. quantity of iron used in ships, especially in steamers, has caused this correction to be attended to now more than formerly, as its effects and magnitude have become more perceptible. The amount of the deviation arising from this local cause varies as the mass of iron changes its position with respect to the compass. When a fore and aft line coincides with the direction of the magnetic meridian, the iron in the ship may be supposed to be nearly equally distributed on both sides of the needle, and its effect in deflecting the needle may be inappreciable. In other positions of the ship with respect to the magnetic meridian, the iron may produce a sensible deflection of the needle; and this deflection or deviation will in general be the greatest when the ship's head points to the east or west. Various methods are used to determine this correction. The one usually adopted is to place a compass on shore, where it may be beyond the influence of the iron of the ship, or any other local disturbing force, and to take the bearing of the ship's compass, or some object in the same direction therewith; at the same time, the observer on board takes the bearing of the shore compass; then, if 180° be added to the bearing at the shore compass, so as to bring it round to the opposite point, the difference between the result and the bearing at ship's compass will be the amount of the deviation of the compass for that position of the ship. Thus, suppose the following bearings are taken when the direction of the ship's head is N. From this it appears the deviation, when the ship's head is north, is 3° easterly. The ship is then swung round one or two points, and a similar observation made; and thus the local deviation found for a second position of the ship. This being repeated for every point or two points of the compass, the deviation is thus known for all positions of the ship. A table, similar to the one below, is then formed, and the courses corrected for this deviation by the following rules; which resemble those already given for correcting for variation. 54. Correct the compass course W.b.S. for deviation & W. (known from table, above). |