CHART (Fr. eharte, (-barter. Lat.. charter, paper. from Gk. ;chary, chart(, paper). A marine ur hydrographic map, exhibiting a portion of a sea or other body of water. with the islands, ad jacent coasts, soundings. currents, etc. (See NI A e. ) Charts are made of eonvenient size and ,eale for the purpose desired and are usually constructed on the principle of Alereator's pro jeetion; charts of the oceans on the gnomonic projection are, however, published by the Hydro graphic Office of the Cnited States Navy for tile comenienee of navigators who wish to follow great-circle courses: and the coast and harbor charts of tile United States, published by the t sited States Coast and tieudetie Survey, are on the polyeonie projection. Charts are con structed with more accuracy than ordinary inaps, and efforts are made to embody in them all the Information possible. The meridians and paral lels of latitude are drawn at convenient intervals, and on harbor charts the exact geographical position of some datum point is given. or more compass-roses (i.e. plans similar to the face of a compass-card) are engraved in the chart where most labelled, or where they will not interfere with soundings or other matters, and show the magnetic variation, while the annual change is recorded in a note. In the case of harbor charts the rise and fall of the tide and 11111i-tidal interval are given; this is important when the tidal range is considerable, for the depths, or soundings. recorded are based upon some particular state of the title, usually that of mean low water. Shoals and shallow waters arc indicated by dotted shading, or stip pled tint, in order that their presence may more readily be noticed. and rocks are marked by erosses or other conventional signs. The sound ing- are expressed in fathoms, or in feet, which evt4r will best serve in the particular locality. and a note under the title states which is used. The character of the bottom is indicated by symbols, giving the kind of material (sand, mud, rock) and its hardness, fineness, color, etc.
Lighthouses. buoys, beacons, and other aids to navigation are carefully shown in their proper position: and, when desirable to do so, the proper courses for entering or leaving a port or channel are indicated by broken lines, alongside which the compass courses are printed. The depths and coa,,t-line in nearly all parts of the world are constantly changing, as are the light houses, buoys, and aids to navigation. This requires unceasing watchfulness on the part of chart-issuing laves. and the correction of charts constitutes more than half the expense connected with their production and issue. Charts already printed and on the shelves of the issuing office are corrected by hand, several draughtsmen being constantly engaged in the work. If, however, the correction in this manner takes more than a few minutes. the copies on shelves are and a new edition printed with the embodied in the engraving. The sources of information for corrections :ire the reports from men-of-war or merchant ships, from lighthouse boards. inspectors of harbors, channels. and Waterway,. new surveys, and 111e These Ore embodied in \otters to Variners and pub lished at short intervals. usually once a week. The information is arranged in the most con venient manner possible for the use of naviga tors, the number of each chart affected by a note being stated. In addition to this, the notices give a list Of charts of which new editions have been published. or which have been extensively
corrected.
For purposes of navigation the character of the projection (i.e. the plan by whieh the curved surface of the earth is represented on a plane) is important. The projection is most used and decidedly the most popular. The meridians are rectilinear and parallel, and this enables the navigator to lay down the compass course as a straight line; and charts on this projection of the sante scale and latitude may be joined at the edges. Neither of these important characteristics is possessed by the polyconie, which is best suited to the delineation of small areas such as harbors of moderate eXtent or certain types of ocean charts. The line of sight, and the great - circle course are nearly straight lines on a polyconie chart, and areas and shapes near the central meridian are un distorted. The saving of distance through fol great-circle areas is very slight except on long voyages, and the conditions of average wind and weather are likely to reduce the value of the saving even then, because the great-eircle arc, when it differs from the rhumb line (q.v.), al ways lies on the polar (and therefore usually the colder and stormier) side of it. Whether it is desirable to follow' a great circle or not an readily be determined by commtring the course as laid down On a great-circle chart (gnomonic projection) with a Wind, weather, and current chart, such as the pilot eh art issued by the 11y drographic Office of tile 'United States Navy. See PILOT CoART, Charts are. now chiefly printed from engraved plates (copper, except ill France, where many soft steel plates are used). for various reasons, toe 'Principal of which is that it facilitates the introduction of new matter ill certain parts with out the necessity of reproducing, the whole. To effect the correction the plate is turned face downward on a smooth anvil or hard surface, and the area to be re-engraved smoothed out and the metal brought up flush with the general surface of the plate by beating on the back over the predetermined area with a round faced ham mer. Preliminary charts, and others of tem porary interest, or of which the details do not change, are largely prod need by photo-lithog raphy. With this process tough, flexible paper cannot be satisfactorily used, and if a photo lithographic chart is to be much handled it re quires backing with cloth.
_\s already stated, there are in the United States two offices. the Hydrographic Office of the Navy. and the Coast Survey, the latter publishing charts (about ROO) of the United States coast only: and the former, charts (about 1500) of the treat Lakes and of foreign territory and seas. The main office of each of these establishments is in Washington, but for the purpose of gathering and disseminating in formation of service to navigators. the llydro graphie Office has branches in all the principal seaports of the rnited Status and several .111 the Great Lakes. It is likely that additional ones will soon be opened at Manila and Honolulu, with observing stations at (thain and Tutuila, when these can he reached by telegraph. In Great Britain tlie llydrographic Offive is under the Admiralty. Charts are also published by nearly all the European powers, and by Brazil, Chile, and Japan; and there are several private makers who issue charts of special localities. The price of Clover/anent charts is very low, barely sullich.nt to cuter the cost of printing and pa per.