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 112 HYDROGRAPHY other. Hydrogen combines with one equiva- lent of oxygen to form hydrogen monoxide or water; with two equivalents to form the di- oxide or oxygenated water, a liquid discovered by Thenard in 1818, and now prepared by chemists for medicinal purposes ; also with one equivalent of nitrogen to form ammonia ; and with one of chlorine to form hydrochloric acid. From his researches on the occlusion of hydro- gen by palladium, Prof. Graham was led to in- fer the existence of an alloy of palladium and hydrogen gas condensed to a solid form, to which he gave the name of hydrogenium. As- suming that the hydrogen enters into the com- bination with the density which it would ex- hibit if solidified in the free state, he calculates, from the observed density of this so-called alloy of palladium and hydrogenium, and of similar alloys containing in addition gold, silver, or nickel, that the density of this hypothetically solidified hydrogen varies between the limits 0-711 and 0-7545 ; mean, 0-733. The presence of hydrogen in the atmosphere of the sun and in the planets has been shown by spectrum analysis. On the sun four lines are attributed to hydrogen. HYDROGRAPHY is the science which, by rep- resentation 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 out- lines 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 at- tached to the sounding lead ; and the tempera- ture of the water is frequently taken as an additional guide to determine the mariner's po- sition. By such sea charts as are now pre- pared and published by the English and French hydrographic offices and by the coast survey of the United States, the risks attending nav- igation have been greatly diminished. (See COAST SURVEY.) Hydrography, as it now ex- ists, 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 im- perfect, 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 determin- ing 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 cen- tury; what little was known being the result of the enterprise of individuals, such as Co- lumbus, Cabot, Drake, and other navigators. The researches of Capt. James Cook of the English navy, which were begun fit Quebec in 1759, when he was master of the frigate Mercury, and were continued for about 20 years, may be considered as the commence- ment of a new era in hydrography. (See COOK, JAMES, and DBS BAREES.) 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 un- timely 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 instru- ments, trained professional hydrographers, and liberal appropriations of money and men, hy- drography has become a recognized branch of public works, and the knowledge of it an ab- solute 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 ac- curate 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