

V H 



MAP. 



4-1 



the pla> ar* amended by lying down the new line* of railway when- 

 erer they ire reprinted. 



On ome of the** government and other large map* of recent 

 production contour- line* have been adopted in the place of shading, to 

 died* the height* of mountains. A good example of the application 

 of contour-lines u afforded by tin- large-scale Ordnance maps of 

 Ireland. On the other hand, tin- Ordnance mane of Wales, and gene- 

 rally the later inch-to-the mile map* of the British surrey, afford very 

 beautiful illustration* 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 mountain*, the varying features of valleys, and, in 

 fact, the general configuration of the surface of a country. 



Of Ut -, also, relief- or model-maps have been published, which show 

 the or -logical and other physical features of countries and ditricU. by 

 pro hieing 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 conic into general use. 



Fr.'in the spherical form of the earth, it u obvious that the divisions 

 and varieties of its surface may be most simply and most accurately 

 represented by mean* of a globe ; and, in order to obtain a correct 

 notion of it* general geographic features, there is no mode of repre- 

 sentation so satisfactory. Large globes, however, are expensive and 

 inconvenient instrument*, 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 con by no contrivance be extended into a plane with- 

 out a distortion of some of it* part*. 



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 denlitpment. 



By finijtrlion is meant the representation of the surface of a sphere 

 on a plane, according to the laws of perspective. By derelopmeiit 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. 



\Ve Khali notice these two principles very briefly, as their mathe- 

 matical investigation more properly belongs to the article PROJECTION. 



There are four methods of spheric projection in general use, the 

 ;,Homonic or central, the orlkoyrajJtic, the itcrcoyrapkic, and the i/lnlmlnr, 

 distinguished from each other by the different positions of the pro- 

 jecting point in which the eye is supposed to be placed. 



The tjHomonic or central projection supposes the eye to be placed in 

 the centre of the sphere, <ind that the various objects to be delineated 

 are 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 



to map* of a limited extent, more especially if they are map* of the 

 polar regions of the globe. In thin case the meridian* will be straight 

 line* radiating from the centre, and the parallels of latitude concentric 

 circle*, whose dutoncn* from the centre will respectively be equal t<> 

 the cotangents of their latitude*. 



In the other cnins of this projection, where the perspective plane is 

 parallel to the horizon, or to any nieri.li m, the construction is rendered 

 tmublenime on account of the parallels of latitude becoming curves of 

 difficult delineation : these cases therefore are -lit into use. 



Ortkoyra):' . In this projection the eye is supposed to 



be at *n infinite distance, so that the visual rays leave the sphei c in 

 parallel lines. The perspective plain- on which a hemisphere is sup- 

 posed to be delineated U the plane { that diameter which i ]HT|-I> 

 dicular to the visual rays: i -.t of the hcmi.-pherc i- 



transferred to this plane by perpendiculars let fall u; '>u it. It will be 

 seen from the figure, that the representation v.ill decrease 



in accuracy with the increase of distance from the centre ; the parti 

 near the circumference being much foreshortened and distorted. 



In a a ixJar ma/i of this projection, the meridians, ox 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 latitude*. 



In an equatorial ma/>, or one in which the equatorial regions of the 

 globe are made to occupy the centre of tho 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 line* parallel t<i tin- 

 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 pol ir di 

 and having their transverse axes coincident with this diameter and 

 equal to it. 



:/rttji/tic /ViyVrf <'>. In this projection the eye is supposed to 

 be placed at the surface of the sphere, and to view the conrive of the 

 opposite hemisphere through the plane of tluat circle, in the pole of 

 which the eye is placed. 



If E be the eye, and A, 0, c the hemisphere to be represented, A, B' 

 c, D will be the plane of projection ; and the position .m this plane of 

 any point of the spherical surface will be indicated by a line drawn 

 from that point through the plane to tho eye. Thus the points K, i., 

 M, K on the sphere will bo transferred to the plane at /, /, in, . 



The advantage* offered by this method of projection have brought 

 it more into use than the methods before mentioned. It is especially 

 calculated for maps of the world, as usually made in two hemispheres, 

 from the circumstance of the representation being leas distorted, and 

 also on account of the meridians and pa.. ecting each other 



at right angles, as they do on the globe. Its construction also is 

 less difficult than others, since all the great circles of the sphere are 

 ircles or straight lines in the projection. The meridian of 20 



\V. is tli le usually selected by Kurdish geographers for the plane of 



projection in these maps of the world, bec.au.-e thi- meridian passes 

 very nearly between the eastern and western continents, which there- 

 fore occupy their ies)>ective hemispheres. 



liliiliuliir Pnijrrtiim. This projection, which is a modification of the 

 Stenographic, was invented by the astronomer De Lahire. who 

 supposed the eye to be placed at a distance from tho sphere equal to 

 the sine of 45 ; that it, if the diameter of the sphere be equal to 200, 

 the distance of tho eye . from the nearest jioint of the circumference 

 would tie 70,^. Some further modification was subsequently deemed 

 desirable, in order that them Hn equator .at 



equal distance*. This condition i- v. i \ nearly fulfilled when the 

 distance of th 



This projection U also much used in maps of the wild. 1 ut to 

 ximplify their construction, the. meridians aud parallels are pi< 



