, or Sea-Chart, a hydrographical or seamap, for the use of navigators; being a projection of some part of the sea in plano, shewing the sea coasts, rocks, sands, bearings, &c. Fournier ascribes the invention of sea-charts to Henry son of John king of Portugal. These charts are of various kinds, the Plain chart, Mercator's or Wright's chart, the Globular chart, &c.

In the construction of charts, great care should be taken that the several parts of them preserve their position to one another, in the same order as on the earth; and it is probable that the finding out of proper methods to do this, gave rise to the various modes of projection.

There are many ways of constructing maps and charts; but they depend chiefly on two principles. First, by considering the earth as a large extended flat surface; and the charts made on this supposition are usually called Plain Charts. Secondly, by considering the earth as a sphere; and the charts made on this principle are sometimes called Globular Charts, or Mercator's Charts, or Reduced Charts, or Projected Charts.

Plain Charts have the meridians, as well as the parallels of latitude, drawn parallel to each other, and the degrees of longitude and latitude everywhere equal to those at the equator. And therefore such charts must be deficient in several respects. For, 1st, since in reality all the meridians meet in the poles, it is absurd to represent them, especially in large charts, by parallel right lines. 2dly, As plain charts shew the degrees of the several parallels as equal to those of the equator, therefore the distances of places lying east and west, must be represented much larger than they really are. And 3dly, In a plain chart, while the same rhumb is kept, the vessel appears to sail on a great circle, which is not really the case. Yet plain charts made for a small extent, as a few degrees in length and breadth, may be tolerably exact, especially for any part within the torrid zone; and even a plain chart made for the whole of this zone will differ but little from the truth.

Mercator's Chart, like the plain charts, has the meridians represented by parallel right lines, and the degrees of the paralleis, or longitude, everywhere equal to those at the equator, so that they are increased more and more, above their natural size, as they approach towards the pole; but then the degrees of the meridians, or of latitude, are increased in the same proportion at the same part; so that the same proportion is preserved between them as on the globe itself. This chart has its name from that of the author, Girard Mercator, who first proposed it for use in the year 1556, and made the first charts of this kind; though they were not altogether on true or exact principles, nor does it appear that he perfectly understood them. Neither indeed was the thought originally his own, viz. of lengthening the degrees of the meridian in some proportions for this was hinted by Ptolemy near two thousand year; ago. It was not perfected however till Mr. Wright first demonstrated it about the year 1590, and shewed a ready way of constructing it, by enlarging the meridian line by the continual addition of the secants. See his Correction of Errors in Navigation, published in 1599.

Globular Chart, is a projection so called from the conformity it bears to the globe itself; and was proposed by Messrs Senex, Wilson, and Harris. This is a meridional projection, in which the parallels are equidistant circles, having the pole for their common centre, and the meridians curvilinear and inclined, so as all to meet in the pole, or common centre of the parallels. By which means the several parts of the earth have their proper proportion of magnitude, distance, and situation, nearly the same as on the globe itself; which renders it a good method for geographical maps.

Hydrographical Charts, are sheets of large paper, on which several parts of the land and sea are described, with their respective coasts, harbours, sounds, flats, rocks, shelves, sands, &c, also the points of the compass, and the latitudes and longitudes of the places.

Selenographic Charts, are particular descriptions of the appearances, spots and maculæ of the moon.

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

For the Construction of Charts, see Geography, Maps, Projection, &c.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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