a sufficiently correct and copious tablc of latitudes and lon: gitudes, or an equivalent to such a table, is one of the helps, which a navigator ought not to be without. A table of this kind is susceptible of the advantage of denoting the situation of places to any required degree of accuracy; but, at the same time, it must be confessed, that names and numbers convey a very imperfect notion of these situations to the imagination, and this purpose is more effectually answered by the use of maps or charts, which, in general, are drawings or pictures of the face of the earth and sea, as they would appear to an eye at a sufficient distance. Those may be called true charts, which are either globes or delineations according to the rules of perspectives; but neither of them is used at sea, because of the few. straight lines they contain. The charts, used at sea, are either plane charts, or Mercator's charts. In the plane chart the meridians and parallels of latitude are right lines, at right angles to each other ; consequently, it has all its meridians parallel to each other, and all its parallels of latitude equal. It is, therefore, useful only in small spaces of the earth's surface, which do not much differ from planes ; such, for example, as a ship’s run in a day, or the extent of a bay or harbour. This kind of chart may be used for several degrees of latitude on each side of the equator, because the meridians are there almost parallel. Mercator's chart is similar to the plane chart, excepting that the degrees of the meridian are not equal, but enlarged toward the poles ; by which contrivance it acquires many valuable propertics. It will be further considered in another place. PROBLEM. PROBLEM. А 44 To construct a plane chart. 44 Draw two right lines parallel to each other across the . paper, one at the top, and the other at the bottom, AB, CD. At right angles to these, draw two other parallel lines, one near the right, and the other near the left extremity of the paper, AC, BD. Divide the two first lines, each into a like number of equal parts, within the points of intersection formed by the crossing of the other two right lines. And divide the other two right lines in like manner into parts, each equal to a part of the first lines. Then the two first lines will represent the extreme parallels of latitude, and the two latter the extreme meridians of the intended chart ; and the subdivisions, on each, will represent degrees, miles, leagues, or any other measure, which may best suit the purpose of the designer. If one of the parallels be supposed to represent the equator, the divisions of the meridians must be numbered thence ; but if not, the divisions must be reckoned from the latitude of VOL. II. ܐ ܝܐ 1 of that paralle!, which is nearest the equator. . And so likewise, if one of the meridians be supposed to represent the first meridian, the divisions of the parallels must be reckoned hence ; but if not, the divisions must be reckcned from the longitude of that meridian, which is nearest the first meridian ; that is to say, as far as 180°, and : ; thence back again Througli every tenth division of the meridians de parallel across the chart ; and through event terib diision of the parallels draw a meridiar * In some coureniot part of the chart draw a piariner's compass, :311 untime the rhumb lines to the extremities of the charts i most charts, the top of the book or paper is med to the consequently the bottom south, the right east, and the left west. Flaces are echiceated or marked on the chart by drawing - parallel throug: the latitude of the place, oil each graduated meridian ; and a meridian through the inngitude of the place, on each graduated parallel : the point of intersection of the parallci and meridian, thus drawn, will give the required situation of the place. Coasts are laid down by marking a sufficient number of points from the known latitudes and longitudes of places on the coast, and the coast itself is draw by the hand through those points. In all sea charts, the line of the coast is gradually shaded off on the land side, to denote the rise of the land above the water. This may be done either with thc pen or Indian ink. On this chart, the situation of a place may be marked, if the bearing and distance from a given place in the chart be know:1. For, draw a line from the given place, paral lel T In the figure, which extends from 10° to 23° of longitude, and from 36° to 44° of north latitude, a meridian is drawn to each degies of longitude, and a parallel to each degree of latitude. lel to that rhumb of the compass, which denotes the bearing, and on this line set off the distance, in parts of the graduated parallel or meridian. The extreme point of the distance marks the place required. The operator will soon observe, that places may be de. lineated on the chart by help of the parallels and meridians already drawn, without having recourse to a multiplicity of other lines. NOTE. Computation is so much more accurate than delineation, and the helps of tables are so many, that charts are seldom used but in coasting navigation. PLANE SAILING, PLANE SAILING is the method of deducing a ship’s place, or of determining things relating to the navigation of a ship, by the principles of the plane chart. The course is the acute angle, formed between the line described, or proposed to be described, by a ship under way, and the meridian, from which she is departing. Distance is the right linc, or rather rhumb, described on any single course. Difference of latitude is the distance between two parallels cr latitude, and is measured on the meridian. It is frequently called northing, or southing. Departure is the distance on the plane chart between two meridians, and is measured on a parallel of latitude. It is frequently called casting, or westing. Difference Difference of longitude is the distance between two meridians, and is measured on the equator. On the plane chart it is the same as departure. It is absolutely necessary, that the navigator should be able to box the compass, as it is called ; that is, to repeat the names of the points in order from memory, either backward or forward, and to tell readily the distance of any point of the compass from the meridian, either in points, or degrees and minutes. For this purpose, the figure of the compass is here drawn. The points being 32, cach is equal to 1° 15'. |