HYDROGRAPHIC OFFICE, United States, an institution established by Act of Congress in 1866 for the improvement of the means for navigating safely the vessels of the navy and mercantile marine. It is attached to the Bureau of Navigation of the Department of the Navy, and prepares the instructions for and receives the results of the ocean and coast sur veys which the naval service is authorized to perform. The accumulations of this office, in the form of charts, sailing directions, and nau tical tables for the use of mariners in the prac tice of navigation and nautical astronomy, have resulted from original naval surveys, from geo graphical and cartographical data gathered from the reports of commanding officers of vessels in the general naval service, from information collected from mariners of all nationalities by the branch hydrographic offices operating in sixteen of the principal ports of the United States, and from the hydrographical informa tion that comes into the custody of the Depart ment of the Navy through the prosecution of surveys by foreign governments. These publl cations relate to all parts of the navigable waters of the globe and form an important asset in the operation of American shipping.
At regular intervals there have also been constructed world-charts showing the lines of equal declination and inclination of the mag netic needle and the intensity of the earth s magnetic force, which have been supplemented by publications of the results of the analytical treatment and the discussion of magnetic ob servations made at a great number of places in all parts of the world, and also charts show ing the telegraphic connections of the world, and showing the tracks for full-powered steamers with the shortest distances in nautical miles. These charts, as well as those issued for purely navigational purposes, are, in nearly all cases, constructed upon the Mercator projection. The exceptions include the circumpolar charts showing the frigid zones and the great circle sailing charts of the several oceans.
A pamphlet of notices to mariners is issued each week, giving information to enable mari ners to amend the publications that are extant from the hydrographic office, by means of de scriptions of newly discovered dangers to navi gation and recently installed or altered lights, beacons and buoys.
The opportunities that surround the seafarer for observing the processes of nature have been made of avail by the hydrographic office in col lecting observations in relation to the physical geography of the sea, which, when collated and digested, are of benefit to the material inter ests of commerce and navigation. For such a field observers are necessary in numbers that come only at the call of nations; and, in order that no man may feel that he has labored in vain, it is necessary to give back to each, to his own profit, the results of the united experience of all. This is the purpose of the pilot charts — to give timely expression, in a graphical form easily comprehended by navigators, of those conditions and facts of experience which, on being taken account of in navigation, will tend to safeguard the lives of seamen and ac celerate transit from place to place. These charts are published monthly for the North Atlantic, Indian, and North Pacific oceans, and at quarter-yearly intervals for the South At lantic and South Pacific oceans. They show the regions of storm, fog and floating masses of ice, the set and rate of ocean currents, the average direction and force of the winds to be expected, the trade-wind limits, the best pas sage routes, the normal isobaric and i thermal lines over the ocean, the variation of the com pass and its change with time and locality, and the positions in which derelict vessels and float ing obstructions to navigation are reported to exist.