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Encyclopedia Britannica - Main :: CHA-CHR |
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CHART (from Lat. carta, charta, a map) . A chart is a marine map intended specially for the use of seamen (for history, see MAP), though the word is also used loosely for other varieties of graphical representation. The marine or nautical chart is constructed for the purpose of ascertaining the position of a ship with reference to the land, of finding the direction in which she has to steer, the distance to sail or steam, and the hidden dangers to avoid. The surface of the sea on charts is studded with numerous small figures. These are known as the soundings, indicating in fathoms or in feet (as shown upon the title of the chart), at low water of ordinary spring tides, the least depth of water through which the ship may be sailing. Charts show the nature of the unseen bottom of the seawith the irregularities in its character in the shape of hidden rocks or sand-banks, and give information of the greatest importance to the mariner. No matter how well the land maybe surveyed or finely delineated, unless the soundings are shown a chart is of little use.The British admiralty charts are compiled, drawn
Earl
Spencer
gentleman
supply of hydrographical information to the whole of the maritime world. The charts prepared by the officers and draughtsmen of the hydrographic office, and published by order of the lords commissioners of the admiralty, are compiled chiefly from the labours of British naval officers employed in the surveying service; and also from valuable contributions received from time to time from officers of the royal navy and mercantile marine. In addition to the work of British sailors, the labours of other nations have been collected and utilized. Charts of the coasts of Europe have naturally been taken from the surveys made by the various nations, and in charts of other quarters of the world considerable assistance hasbeen received from the labours of French, Spanish, Dutch and American surveyors. Important work is done by the Hydro-graphic Office of the American navy, and the U.S. Coast and Geodetic Survey. The admiralty charts are published with the view of meeting the wants of the sailor in all parts of the world. They may be classed under five heads, viz. ocean, general, and coast charts, harbour plans and physical charts; for instance, the Indian Ocean, the Mediterranean, approaches to Plymouth, Plymouth Sound and wind and current charts. The harbour plans and coast sheets are constructed on the simple principles of plane trigonometry. by the surveying officers. (See SURVEYING: Nautical.) That important feature, the depth of the sea, is obtained by the ordinary sounding line or wire; all soundings are reduced to low water of ordinary spring tides. The times and heights of the tides, with the direction and velocity of the tidal streams, are also ascertained. These MS. charts are forwarded to the admiralty, and form the foundation of the hydrography of the world. The ocean and general charts are compiled and drawn
As certain abbreviations are used in the charts, attention is called to the " signs and abbreviations adopted in the charts published by the admiralty." Certain parts of the world are still unsurveyed, or not surveyed in sufficient detail for the requirements that steamships now demand. Charts of these localities are therefore drawn in a light hair-line and unfinished manner, so that the experienced seaman sees at a glance that less trust is to be reposed upon charts drawn in this manner. The charts given to the public are only correct up to the time of their actual publication. They have to be kept up to date. Recent
The charts are supplemented by the Admiralty Pilots, or books of sailing directions, with tide tables, and lists of lighthouses, light vessels, &c., for the coasts to which a ship may be bound. The physical charts are the continuation of the work so ably begun by Maury of the United States and FitzRoy of the British navy, and give the sailor a good general idea of the world's ocean winds and currents at the different periods of the year; the probable tracks and seasons of the tropical revolving or cyclonic storms; the coastal winds; the extent or months of the rainy
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