On some of these government and other largo maps of recent production contour-lines have been adopted in the place of shading, to indicate the heights of mountains. A good example of the application of contour-lines is Afforded by the largosesle Ordnance maps of Ireland. On the other hand, the Ordnance maps of Wales, and gene rally the later inch-to-theanile maps of the British survey, afford very beautiful illustrations of the plan of graduated shading to a carefully considered scale for orological delineations. But much yet remains to be accomplished, at least in ordinary maps, for the delineation of the true forms of mountains, the varying features of valleys, and, iu fact, the general configuration of the surface of a country.
Of lats, also, relief- or modebmape have been published, which show the orological and other physical features of countries and districts, by producing the various elevations in bold relief of proportionate heights, by means of pressure upon softened and specially prepared paper. Some beautiful specimens have been produced ; but. when executed with accuracy, the process is too costly to come into general use.
From the spherical form of the earth, it is obvious that the divisions and varieties of its surface may be most simply and most accurately represented by means of a globe ; and, in order to obtain a correct notion of its general geographic features, there is no mode of repre sentation so satisfactory. Large globes, however, are expensive and inconvenient instruments, and small ones, by not admitting sufficient detail, are for most geographic purposes entirely useless. Hence we see the eminent utility of maps, notwithstanding the imperfections which necessarily accompany such a mode of representation; for a spherical surface can by no contrivance be extended into a plane with out a distortion of some of its parts.
The methods adopted in the construction of maps are as various as the taste and judgment of geographers themselves, but they may all be referred to two principles—namely, projection and siccdopment.
By projection is meant the representation of the surface of a sphere on a plane, according to the laws of perspective. By derelopment is to be understood the unfolding or spreading out of the spherical surface on a plane. This, however, first supposes the sphere to be converted into a cone or a cylinder,—these being the forms, portions of which most resemble portions of a sphere, and which at the same time are susceptible of the required development.
We shall notice these two principles very briefly, as their =the matinal investigation more properly belongs to the article PROJECTION.
There are four methods of spheric projection in general use,—the gnomonic or central, the orthographic, the stercographic, and the globular, —distinguished from each other by the different positions of the pro jecting point in which the eye is supposed to be placed.
The gnomonic or central projection supposes the eye to be placed in the centre of the sphere, sad that the various objects to be delineated aro transferred from the sphere to a plane, which is a tangent to its surface. The entire hemisphere can never be represented by this pro jection, since the circumference which terminates it is on a level with the eye, and is therefore parallel to the plane of projection. This method is chiefly used in dialling, but may be advantageously applied in accuracy with the increase of distance from the centre ; the parts near the cirmunference being much foreshortened and distorted.
In a a polar map of this projection, the meridians, as in the gnomonic maps, will be radii, and the parallels concentric circles; these circles, however, will have their distance from the centre equal to the cosines, and not to the cotangents of their respective latitudes.
In an equatorial map, or one in which the equatorial regions of the globe are made to occupy the centre of the map, the plane of pro jection coincides with the plane of one of the meridians. In this case the latitude circles will be projected in straight lines parallel to the equator, which is also a straight line, and will vary in distance from it according to the sines of their respective latitudes. The meridians will be portions of ellipses intersecting the equator in points similar in position to the intersecting points of the parallels on the polar diameter. and having their transverse axes coincident with this diameter and equal to it.
,Ftereographic Projectiol.—In this projection the eye is supposed to be placed at the surface of the sphere, and to view the concave of the opposite hemisphere through the plane of that circle, in the pole of which the eye is placed.