MAP-MAKING. The mechanical execu tion of a map is simply an example of engrav ing on steel, copper, stone, wood, or zinc ; but there are a few expressions relating to the projection or planning of a map which it is useful to know.
The methods adopted in the construction of maps are as various as the taste and judg ment of the geographers themselves, but they may all be referred to two principles, viz. Projection and Development. By Pi ejection is is meant the representation of the surface of a sphere on a plane, according to the laws of perspective. By Development is to be under stood the unfolding or spreading out of the spherical surface on a plane. This however first supposes the sphere to be converted into a cone Qv a cylinder—these being the forms, portions of which most resemble portions of sphere, and which, at the same time, are sth ceptible of the required development.
The Gnomonic or Central Projection sur poses the eye to be placed in the centre c the sphere, and that the various objects to b delineated are transferred from a sphere to plane, which is a tangent to its surface. Th Maps of the Stars, prepared by the Societ. for the Diffusion of Useful Knowledge, are oi this projection. In the Orthographic Projec distance, so that the visual rays leave thi will decrease in accuracy with the increase o distance from the centre ; the parts near tin circumference being much foreshortened an c distorted. In the Stereographic Projection opposite hemisphere through the plane of that circle, in the pole of which the eye is placed. It is especially calculated for maps of the world, as usually made in two hemis pheres, from the circumstance of the repre sentation being less distorted, and also on account of the meridians and parallels inter setting each other at right angles, as they do on the globe. In the Globular Projection, the
eye is supposed to be placed at a distance from the sphere equal to the sine of 45'; that is, if the diameter of the sphere be equal to 200, the distance of the eye from the nearest point of the circumference would be 70-k. Some modification was subsequently deemed desirable, in order that the meridians might intersect the equator at equal distances. This condition is nearly fulfilledwhen the distance of the eye is 59i, the diameter being 200. This projection is also much used in maps of the world. In the Conical Projection, the sphere is supposed to be circumscribed by a cone, which touches the sphere at the circle intended to represent the middle parallel of the map ; the points on the sphere being projected on the cone, the latter is then con ceived to be unrolled or developed on a plane surface. In the Cylindrical Projection, the sphere is supposed to be circumscribed by a cylinder instead of a cone. The peculiarity of this method is, that the meridians, as well as the latitude circles, are projected in parallel straight lines. In Mercator's Projection; so largely used by mariners, the degrees of lati tude on the chart are made to increase to wards the poles, in the same ratio as they decrease on the globe ; by which means the course which a ship steers by the mariner's compass becomes on the chart a straight line; the various regions of the map, however dis a torted, preserve their true relative bearing, and the distances between them can be accu rately measured.