Page:The American Cyclopædia (1879) Volume XI.djvu/148

 136 MAP distortion increasing from the centre, it is less than by some of the other methods. The stereographic method is much used for the maps of the world drawn in two hemispheres; and the meridian of 20 W. from Greenwich is usually selected for the plane of projection, because this throws the two great continental divisions of the earth into their respective hemispheres. In the central or gnomic pro- jection, the eye is supposed to be at the centre of the earth, and the objects upon the surface are projected upon a plane which is a tangent to its surface. This method is obviously ap- plicable to maps of a limited extent only ; and except for maps of the polar regions, where the parallels of latitude are concentric circles, and the meridians are straight lines, they are troublesome to execute on account of the ir- regular curves the parallels assume. In the globular projection, the eye is supposed to be at a distance from the sphere equal to the sine of 45; or, the diameter being 200, this dis- tance is 707. In order, however, that the meridians may intersect the equator at equal distances, the distance for the eye is generally fixed at 69^, the diameter being 200. Maps are also constructed in which the meridians are represented by arcs of circles cutting the equatorial diameter at equal distances, and the parallels by arcs of circles cutting the polar diameter at equal distances. These maps are not projections, and founded upon no geomet- rical principle which can be of service in their use ; nevertheless they give a very good repre- sentation of the forms and relations of areas, and are of very simple construction. They are called globular maps, but must not be con- founded with maps constructed upon the prin- ciple of globular projection, mentioned above. Another method of map making is based upon the principle called development, which is a mode of projecting the forms upon the surface of the earth upon the inner surface of a cone or of a cylinder, which is supposed to envelop the earth and touch it only around the circle which is to be the middle latitude of the map. The points on the earth's surface being projected by other lines drawn through them from the centre, the inner surface of the cone or cylinder is afterward supposed to be unrolled or developed, and thus present the various objects upon a plane surface. Those situated nearest the middle latitude will be most correctly represented. In the use of the cylinder the latitude circles and meridians ap- pear as parallel straight lines, and thus most correctly represent for nautical purposes the angles at which they are cut by objects moving over the surface on any other lines. This principle is in part the foundation of the pro- jection known as Mercator's, and applied by him to charts for navigators, in which the cor- rect bearings of objects upon the surface are of more importance to determine than the true [ figures of countries. Still other principles are employed in constructing maps, according to the special purposes for which they are de- signed. In maps of small areas, the figure of the earth may be neglected, and the positions and forms of bodies be represented as if the surface w.ere itself a plane. Some have special objects in view, as the delineation of the coast lines, channels, shoals, reefs, lighthouses, &c., hence called hydrographic maps or charts; others are intended to show the political divi- sions of states, counties, and towns ; and others, designated topographical maps, to represent the natural features of a country, as its mountains, hills, rivers, plains, &c., for all of which certain conventional signs are adopted. Maps have also been constructed to represent the courses of the winds and of oceanic currents over the surface of the earth ; to designate the position of the isothermal lines ; to indicate the geolo- gical formations found in different regions; and others to indicate the flora and the fauna of different countries. In the construction of geographical maps covering large areas, the principal places are located according to their latitudes and longitudes, and the lines of coasts and of countries, roads, &c., are plotted from the most exact surveys that have been made. Those which have been conducted under gov- ernment patronage have furnished the mate- rials for the best maps, and these are constant- ly improving as new materials are collected. Of the United States, the most complete maps are those of the state of Massachusetts made by order of the legislature, of the coast survey under the general government, Whitney's sur- vey map of California, and Clarence King's survey map of the 42d parallel. The great lakes, more especially on the Canadian side, have been surveyed and mapped with great accuracy by Lieut. Bayfield of the royal army. Maps of the Spanish provinces in America have been made by the Spanish hydrographical depot in Madrid ; and Brazil and other South American states have executed maps of their territories. The ancient Egyptians had some knowledge of maps, as Sesostris caused the territories he possessed and had conquered to be represented upon tablets for the instruction of his people; and the Israelites appear to have acquired the same knowledge, from the record, in Josh, xviii. 6, of a map of the coun- try being ordered by that lawgiver. The first map of the world, as known to the ancients, is said to have been made by Anaximander the Milesian. Herodotus makes mention of maps constructed by the Persians in the time of Darius, and of one by Aristagoras of Mile- tus. Eratosthenes introduced the lines of lati- tude and longitude, and the use of these was established by Hipparchus upon a mathemati- cal principle. Still, for want of exact surveys, and owing to the dependence of geographers upon the reports of travellers and their itine- raria picta, or painted itineraries, the maps afterward made were extremely inaccurate. Even those of Strabo and Ptolemy, of which those of the latter were for centuries the chief