Hydrography, is the science which, by representation of the figure of the bottom of the ocean and its tributaries by means of soundings, by observations of tides and currents, and by investigations of the winds and their action and of the law of storms, aims to diminish the risk attending the navigation of dangerous waters. The results of these investigations are shown upon charts, which give the outlines of the coasts and harbors, the depths of water in the navigable channels, the rocks and shoals with the soundings upon them, and various tidal and magnetic information. In the course of the investigations specimens of the bottom are also obtained by apparatus attached to the sounding lead; and the temperature of the water is frequently taken as an additional guide to determine the mariner's position. By such sea charts as are now prepared and published by the English and French hydrographic offices and by the coast survey of the United States, the risks attending navigation have been greatly diminished. (See Coast Survey.) Hydrography, as it now exists, belongs to modern times, although various rude attempts at hydrographic examinations and the construction of sea charts were made in early times.

The invention of charts for mariners is commonly ascribed to Henry the Navigator (1394-1460), although earlier ones exist. Of necessity such were rude and imperfect, the size and even the true shape of the earth being then unknown, the log for measuring nautical miles not in use, the only instrument for determining latitude being the sea astrolabe, and none existing for determining the longitude. Little was accomplished through national instrumentality toward the improvement of our knowledge of the sea and its tributaries until the middle of the 18th century; what little was known being the result of the enterprise of individuals, such as Columbus, Cabot, Drake, and other navigators. The researches of Capt. James Cook of the English navy, which were begun at Quebec in 1759, when he was master of the frigate Mercury, and were continued for about 20 years, may be considered as the commencement of a new era in hydrography. (See Cook, James, and Des Barres.) The success of the English captain excited the rivalry of the French; and in 1785 La Perouse was placed in command of an expedition consisting of two frigates, with a corps of scientists, and sent to continue the work which Cook's untimely fate had left unfinished.

They were never heard from after their departure from Botany bay; but La Perouse had sent home from there duplicates of the journals and charts of his discoveries up to the date of his arrival. D'Entrecasteaux's unsuccessful expedition in search of him in 1791 gave rise to a text book on marine surveying by his navigating officer, Beautemps-Beaupre, published as an appendix to the narrative of D'Entrecasteaux's voyage (1808). This, with the exception of Alexander Dalrymple's "Essay on the most Commodious Method of Marine Surveying" (1771), was the first treatise published in a practical shape. About the time of its publication Beautemps-Beaupre took charge of the survey of the French coast, and trained a corps of hydrographers, who formed the nucleus of a body of scientific engineers to be furnished to future expeditions for surveying and exploration. Spain has also done a great deal for hydrography, although in a more indirect way. The legal provision for the examination of officers of the mercantile marine as to their competency to navigate a vessel, before promoting them, has given a high reputation to its merchant service; and the nautical information obtained from that source has been found exceedingly valuable.

Her example has of late years been followed by almost every nation having much commerce. But in our own times, with improved instruments, trained professional hydrographers, and liberal appropriations of money and men, hydrography has become a recognized branch of public works, and the knowledge of it an absolute necessity to the complete seaman. Re-connoissances of large extents of coast have been made by men trained to the practice of the science, with such success as to be scarcely capable of correction by the results of detailed surveys. In the latter the aid of geodesy (by which the positions of points on shore are accurately determined) is called in; and no such examination is considered complete or accurate unless it depends upon triangulation. (See Coast Survey, vol. iv., p. 757.) Great Britain, France, Spain, the United States, and other nations have now their hydrographic offices as established branches of government; and under the direction of these departments close and accurate surveys are made of the home coasts, and their surveying vessels frequent all parts of the globe, and penetrate seas hitherto almost unknown, mapping the limits of harbors, determining with precision the geographical position of headlands and entrances, and of rocks, shoals, and sands, many of them hitherto unknown.

In this science "England is far in advance of all other nations. Not content with a most complete and admirable survey of her own coasts, she has extended her work to all of her possessions and to the coasts of foreign nations. Many eminent surveyors are numbered among her naval officers; but it is probable that few have done so much or displayed so much zeal and devotion to the science as the late Admiral Beaufort, so long at the head of the hydrographic office of the admiralty. His surveys were sometimes actually carried on at his own expense. Much importance is attached to the results expected from the scientific cruise of the British ship Challenger, which at the present time (1874) is engaged in a voyage around the world, probably the most important of its kind ever undertaken. She carries a large number of men familiar with almost all the branches of science and art, whose labors, it is hoped, will be productive of much information in natural science and in marine surveying and deep-sea dredging. Although surpassed by England in the number and completeness of her foreign surveys, the hydrographic work on our own coasts is unequalled for accuracy and rapidity of execution.

Under the charge of the coast survey of the United States it has progressed in company with the trigonometrical and topographical work of that service; and it is safe to assert that the completed charts of the coast and the various harbors stand alone in the annals of surveying for beauty of execution, accuracy, and completeness of detail. A large corps of skilled professional hydrographers are constantly employed prosecuting the surveys of the numerous harbors on the Atlantic, Pacific, and gulf coasts; and others are engaged in deep-sea explorations along the course of the Gulf stream, in the gulf of Mexico, and on the coasts of California and Oregon. These deep-sea expeditions have been especially useful in determining the routes suitable for submarine cables, several of which have been laid over lines previously sounded and surveyed by officers belonging to the coast survey. One of the most successful hydrographic expeditions of modern times was that undertaken between 1851 and 1853 under the auspices of the coast survey of the United States, by Lieut, (now Rear Admiral) James Alden of the navy, in the schooner Ewing and steamer Active. More than 1,300 m. of the Pacific coast was explored, from lat. 32° 30' to 48° 20' N, and the geographical positions of all the prominent headlands and of the entrances to the harbors were determined by astronomical observations, from the southern boundary of the United States to the strait of Fuca; lines of soundings were carried along the coast throughout its entire length, and hydrographic recon-noissances made of most of the harbors, with accurate views of the different entrances and of prominent points on the coast; and subsequent careful detailed surveys, based upon accurate geodetic determinations, have failed to change the results of this work in any important particular.

The immediate result of this recon-noissance was the publication of a chart of the Pacific coast for the use of mariners, and subsequently of a marine directory, which has since been elaborated and published as a "Coast Pilot of the Pacific Coast of the United States." - The method of hydrographic surveying, as now practised both in this country and in Europe, is as follows: 1. Reconnoissance, as, for instance, the hydrographic survey of a harbor on a foreign coast, or any place where accurate geodetic information cannot be obtained. The hydrographer, obliged himself to make all the determinations of points on shore and the outlines of the coast, applies the principles of geodesy and topography, but of course in a comparatively rude manner. A base line may be measured, if on land, in the ordinary way; but if the working ground is so far from shore as to render points on shore useless (as is sometimes the case in surveys of shoals off a low and flat coast), or if the coast is occupied by an enemy, a base line is sometimes measured by anchoring a boat at each end of it, and noting the interval between the flash of a gun fired from one boat and the report as heard at the other.

But this very rude method is only admissible where no other is possible. "Where the surface to be surveyed is small, good results have been obtained from a base line measured by a cord, the two ends being marked either by boats or buoys. Signals are erected at each end of the base line and on prominent points along the shore, the latter being determined by horizontal angles measured from each end of the base line. Not only the angle between each end of the base and each signal is measured, but the angles between the different signals themselves; and the triangles thus formed are either computed by trigonometry or platted by intersections upon the chart. The latitude and longitude of some prominent points are also determined. The outlines of the coast or harbor are drawn between intermediate points determined by horizontal angles, and the chart is then ready for platting the sounding lines. Next, a tide gauge is erected. This is generally a plain staff, graduated to half feet; and by continuous observations of the rise and fall of the tides, and of the times of high and low water, the hydrographer obtains an approximate establishment for the port, and also the means of correcting his soundings for the rise of the tide, which is called "reducing them to the level of low water." The shore line having been rudely determined, and such natural and artificial features mapped as may be considered necessary, a boat is started from any point in the harbor to run the lines of soundings.

The boat is steered on a certain course, and soundings are taken at intervals as nearly regular as possible. These soundings, together with the time at which they are taken and the horizontal angles for position, are recorded. The end of the line is also determined by angles; and the boat is then started on a new line. Thus the harbor or bay is crossed and recrossed by lines of soundings intersecting each other in numerous places; and these soundings, reduced to low-water level and laid down upon the chart, show the depth at low water not only in the channel but on the various shoals. 2. Deep-Sea Soundings. In this kind of hydrography the position of the vessel is determined from time to time by careful and numerous observations of the sun and stars, and by dead reckoning. The line used has recently been successfully replaced by a wire, and the lead or shot at the end of it is so arranged as to be detached on striking the bottom. An instrument called an indicator is attached to the sounding line, which, by means of revolving disks put in motion by a screw-propeller wheel, registers the depths to which it descends; when relieved of the weight of the lead, it is thrown out of gear and drawn up.

The line is drawn in by a reel worked by a small steam engine; and by means of all these appliances soundings are taken at great depths with a rapidity and accuracy utterly unknown until of late years. Specimens of the bottom are obtained by means of specimen cups attached to the sounding line, or by the dredge. The best indicators now in use are those of Trowbridge and Brooke, the latter gentleman's having given thus far the best results. 3. Hydrographic Surveys. The process in a detailed survey is similar to that in a reconnoissance, but more elaborate. The hy-drographer is furnished with the positions of numerous points on shore and with a map of the shores of the harbor in detail, on a scale to suit his own work. Upon this map are platted the points furnished him from the geodetic survey; and upon it he also constructs his lines of soundings. Usually two, and sometimes three officers are employed in each boat in running the lines, the advantage of this arrangement being that the two angles necessary to determine the position of the boat can be taken at the same moment by two observers without stopping the boat. Sometimes, especially where the work lies at a distance from the shore, two observers are placed on prominent points on shore, each with a theodolite.

At stated intervals the surveying boat or vessel hoists a ball or flag, when both observers direct their instruments to her, and upon the instant of its being lowered measure the angle between the boat and some fixed point. The intersection of their two lines of sight when platted upon the chart gives the position of the boat. The lines of soundings are run more closely than in reconnoissance, and as far as possible are made to cross each other at right angles. Tidal observations are made to tenths of a foot; and the box gauge, and at certain central points the self-registering gauge, are used. (See Coast Survey, vol. iv., p. 762.) The surveying parties, from the chief to the leadsman, are specially trained for the work, and the resulting accuracy of such a survey is correspondingly great. - Physical hydrography investigates the laws of the formation of shoals, the effect upon harbors and channels of the tidal currents, of the extension of wharves, and of the dumping of earth and ballast; and endeavors to provide remedies for the changes which injure a harbor, and to suggest means for improving the channels.

This branch of the science has of late years attained to great importance both in Europe and the United States, and the researches of those who have devoted themselves to its study have resulted in incalculable benefits to commerce. (See Coast Survey, vol. iv., p. 761.) In regard to currents, and other hydrographic details, see Atlantic Ocean, and Dredging (Deep-Sea).