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 sea--with 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 and issued by the
hydrographic office. This department of the admiralty was established
under Earl Spencer by an order in council in 1795, consisting of the
hydrographer, one assistant and a draughtsman. The first hydrographer
was Alexander Dalrymple, a gentleman in the East India Company's civil
service. From this small beginning arose the important department which
is now the main source of the 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 has been
received from the labours of French, Spanish, Dutch and American
surveyors. Important work is done by the Hydrographic 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 cla