CHART, or hydrographical map, in na vigation, is a representation, in piano, of a part, or of the whole, of the water on the surface of the globe, and the adjacent coast. There are various kinds of charts, as Globular, Plane, Mercator's, &c.
Caster, globular, is a projection, so call.
ed from the conformity it bears to the globe itself. This projection was pro posed by Senex, in which the meridians are inclined, the parallels equidistant and curvilinear, and the rhomb-lines real spi rals, as on the surface of the globe. From this last property, it is evident it can be of very little use in navigation ; as a map, however, it has its advantages.
Construction of Charts.
1. Of the plane chart.—The number of degrees of latitude which the chart is in tended to contain, and the extent from east to west being fixed upon, a line is to be drawn near the side or end of a sheet of paper, in length equal to the whole length of the chart from north to south ; and this line is to be divided into degrees, and numbered accordingly. From each end of this line perpendicu lars are to be drawn, and made equal to the intended extent of the chart from east to west, and their extremities are to be joined by a straight line. If the chart is to commence at or near the equator, and to extend only a few degrees of latitude, the divisions of the parallels may be equal to those of the meridian ; but if the chart be gins at any considerable distance from the equator, it will conduce to accuracy, to make the length of each degree ofthe pa. rallel equal to the co-sine of the mean la titude, the radius being 60 minutes ; or the extreme parallels may be divided ac cording to the above proportion, and in that case it will become a reduced chart. Meridians and parallels are there to be drawn at convenient distances.
A scale is now to be made of stiff paper or pasteboard, equal in length to the ex. tent of the chart from east to west, and divided and numbered accordingly. By this scale, the positions of those places contained within the limits of the chart are very easily laid down, by placing the divided edge of the scale over the latitude of the given place ; and under the given longitude, a mark being made will repre sent the position of the place on the chart.
A compass is to be inserted in any con venient place of the chart, an arrow sheav ing the direction of the flood tide or cur rent. The times of high water at full and
change are to be marked in their proper places, expressed in Roman characters ; sounding and quality of the ground at bot tom, the leading marks to avoid dangers, &c.
II. Of a difercator's Mercator's chart, for anygiven portion of the of the globe, is constructed as follows; the limit of the proposed chart is first to be determined ; that is, the number of de grees of latitude and longitude which it is to contain, and the degree of latitude and longitude of its commencement. Find the meridional parts answering to each degree of latitude, within the intended limits of the chart, and take the difference between each, and that corresponding to the least degree of latitude in the chart; and reduce these differences ito degrees by dividing by 60.
A parallel, representing that of the least latitude, is to be drawn ; upon which the number of degrees in the proposed differ ence of longitude, from a scale of equal parts, is to be laid off, and divided into de grees, and smaller portions of, if conve nient, and numbered at each fifth or tenth degree. From each end of this pa rallel a perpendicular is to be drawn, and made equal to the difference of the meri dional parts of the extreme latitudes taken from the divided parallel ; and the ends of these meridians are to be joined by a straight line, which will represent the other extreme parallel, and which is to be divided and numbered in the same manner as the first drawn parallel ; the meridians are then to be divided into de grees, and numbered at every fifth or tenth degree. Take the meridional dif ference of latitude between the begin ning of the chart, and the next fifth or tenth degree of latitude from the divided parallel, and lay it off from the first paral lel on each of the scale meridians, and join these points by a straight line. In like man ner the meridional difference of latitude, answering to each successive interval of five or ten degrees, is tobe taken from the first drawn parallel and laid off, and the corresponding parallels are to be drawn and numbered accordingly, and the inter mediate spaces are to be subdivided. If the chart is upon a large scale, the meridional difference oflatitude answeringto eachde gree is to be laid off from the leastparallel.