or Sea-chart, an hydrographical map, or a projection of some part of the earth's superficies in plano, for the use of navigators.

Charts differ very considerably from geographical or land-maps, which are of no use in navigation. Nor are sea-charts all of the same kind, some being what we call plane-charts, others mercator-charts, and others globular charts.

Plane Chart, is a representation of some part of the superficies of the terraqueous globe, in which the meridians are supposed parallel to each other, the parallels of latitude at equal distances, and consequently the degrees of latitude and longitude everywhere equal to each other. See Plane Chart.

Mercator's Chart, is that where the meridians are straight lines, parallel to each other, and equidistant; the parallels are also straight lines, and parallel to each other: but the distance between them increases from the equinoctial towards either pole, in the ratio of the secant of the latitude to the radius. See Navigation.

Globular Chart, a meridional projection, wherein the distance of the eye from the plane of the meridian, upon which the projection is made, is supposed to be equal to the sine of the angle $45^\circ$. This projection comes the nearest of all to the nature of the globe, because the meridians therein are placed at equal distances; the parallels also are nearly equidistant, and consequently the several parts of the earth have their proper proportion of magnitude, distance, and situation, nearly the same as on the globe itself. See Globular Projection.

Hydrographic Charts, sheets of large paper, wherein on several parts of the land and sea are described, with their respective coasts, harbours, sounds, flats, rocks, shelves, sands, &c. together with the longitude and latitude of each place, and the points of the compass. See Mercator's Chart.

Selenographic Charts, particular descriptions of the spots, appearances, and maculae of the moon. See Astronomy, p. 63. and 140.

Topographic Charts, draughts of some small parts of the earth only, or of some particular places, without regard to its relative situation, as London, York, &c.