What has been said will be rendered familiar to the learner by the following examples : In the last example, it is evident, that as the difference of latitude is more than the latitude left, the ship must have crossed the equator, and consequently has come into south latitude. Note. When one of the places has no latitude, or is on the equator, the latitude of the other place is their difference of latitude. In the last example, the ship has crossed the opposite meridian, and therefore has come into a longitude of a different name PLANE SAILING. PLANE SAILING is the art of navigating a ship upon principles deduced from the supposition of the earth's being an extended plane, on which the meridians are all parallel to each other. A map of the several parts of the earth, constructed upon these principles, is called a PLANE CHART. When the parts of the earth are thus delineated on a plane, it is easy to see the track by which a ship may go from one place to another, and also what angle this track makes with the meridian.* Ships at sea are kept in this tract by means of an instrument called the mariner's compass. The MARINER'S COMPASS is an artificial representation of the horizon of any place. It consists of a circular piece of paper (see Plate VI. fig. 1), called a card, divided (like the horizon) into 360 degrees, or 32 points. This is fixed on a piece of steel, called a needle, to which the magnetic virtue has been communicated by means of a loadstone, which has the property of pointing steadily towards the north, and carrying the card with it, when turning freely on a pivot or any thing to support it. Thus all the points of the card will be directed towards their corresponding points of the horizon; † consequently, by help of the compass, a ship may be kept in any proposed track or course. The COURSE is the angle which the line described by a ship makes with the meridian, being sometimes reckoned in points, half points, &c., and sometimes in degrees. DISTANCE is the way or length a ship has gone on a direct course in a given time, The method of measuring this distance by the log will be explained hereafter. DIFFERENCE OF LATITUDE is the distance which the ship has made north or south of the place sailed from, or the portion of the meridian contained between the parallels of latitude sailed from and come to. DEPARTURE is the east or west distance a ship has made from the meridian, or the whole easting or westing made by the ship. If a ship sails due north or south, she sails on a meridian, makes no departure, and her distance and difference of latitude are the same. If she sails due east or west, she goes on a parallel of latitude, makes no difference of latitude, and her departure and distance are the same. The difference of latitude and the departure make the legs of a right-angled triangle, the hypotenuse of which is the distance the ship has sailed; the perpendicular is the difference of latitude counted on the meridian; the base is the departure, which is easting or westing counted from the meridian; the angle opposite to the base is the course, or angle that the ship makes with the meridian; and the angle opposite the perpendicular is the complement of the course, which being taken together, make always 8 points or 90 degrees. In constructing figures relating to a ship's course, let the upper part of the paper, or what the figure is drawn upon, always represent the north; the lower part will be the south; the right hand east, and the left west. Draw the north and south line to represent the meridian of the place the ship sails from; then, if the ship's course is to the southward, mark the upper end of the line for the place sailed from; but if the course is northward, mark the lower end for that place. When the course is easterly, describe the arc, and lay off the course and departure on the right-hand side of the meridian; but when westerly, on the left-hand side. When the course is given in degrees, they must be taken from the protractor, oi from the line of chords; but when in points, from the line of rhumbs, and must always be laid off upon the arc, beginning at the meridian. The method of calculating this angle on the true principles of sailing on the spherical surface of the earth, will be given hereafter. + It is here supposed that the needle points to the true north, but if it varies therefrom, allowance mus When the course is given in points, the log. sine, log. cosine, &c., may be found in Table XXV., otherwise in Table XXVII. In all cases, where the complement of course, or cosine, &c. is used, the degrees of points put down are the course itself, but the logarithms belonging to the complemen or cosine, &c., of that course are taken. A Table of the Angles which every Point of the Compass makes with the Meridian. In the following Table, the Rules for solving the various Cases of Plane Sailing are collected. PLANE SAILING. |