Page:The New International Encyclopædia 1st ed. v. 10.djvu/433

* HYDROGKAPHY. 377 HYDROGRAPHY. up to the time of his arrival at Botany Bay. The navigating ofiBcer of the expedition, Beau- temps-Beaupre, sent out to search for La POrouse in 1701 under the command of D'Entrecasteaux, wrote a work on marine surveying, which was published in an appendix to the narrative of the voyage (180S). This, however, had been preceded by Alexander Dalrvmple's essay on marine sur- veying, published in 1771. Beautemps-Beaupre , was placed in charge of the sur-ey of the French coast, where he trained a number of hydrogra- phers. the commencement of a corps of engineers tor future exploration and surveying. The work done by these early English and French naviga- tors was imitated by those of other nations, and now, as previously stated, practically every civilized nation has its hydrographie office. • Collection of Hydrograi'hic Data. The sources from which hydrographie data are gath- ered are numerous. One of the most important is the system of surveys made directly by the Hydrographie Office (q.v.). Some of the objects of these surveys are the determination of deptlis for mapping and navigation purposes ; the loca- tion of Ijuoys, rocks, signals, etc.; the location of channels, the directions and velocities of cur- rents, and the determination of the changes in the same, the measuring of the cross-sections of streams, the mean velocities of the water across such sections, and the slope of the water surface, and the determination of the quantity of sedi- ment carried in suspension, the vclume of scour or fill on the bottom, or of the material removed by artificial means as by dredging. The fixed points of reference for the survey are usually on shore, but sometimes buoys are anchored off shore and used as points of reference. The depth of water is determined by taking sound- ings along certain definite lines, these lines being established from the reference points just men- tioned in coast and harbor work, and by the sextant and mariner's compass in deep-sea w'ork. The operation varies in its nature, but consists commonly in rowing a boat at uniform speed and making soundings at regular inter-als of time. The steersman keeps the boat true to the line by starting from a known point and steering direct- ly toward another known visible point. In deep- sea sounding the ship keeps to a compass course, the latitude and longitude being established at suitable intervals by the ordinary means. In a harbor survey the soundings are generally taken along two sets of parallel lines, one set crossing the other at approximatelv right angles, and thus dividing the harbor area into checker- hoard divisions. In river work the soundings are usually made along lines crossing the streams transversely at regular intervals. Numerous modifications of these two methods are practiced. The means used for making the sounding or measuring depth ranges from a long graduated rod in shallow waters to the deep-sea lead. I See Lead, SoiNniNO.) To determine the direction and velocities of currents various mean? are employed, the most perfect of which are in- genious instruments known as current-meters (q.v.). In these devices a small water-wheel or screw propeller is operated by the current, and registers the number of revolutions made. As the wheels are regulated to make a certain num- her of revolutions at each speed of current, the registered record shows by a simple calculation the velocity of the current at the point where the meter was sunk. For collecting samples of the subsurface water instruments are used which consist essentially of a small water-tight vessel which can be tightly closed below water, and thus enable a sample to be brought to the surface by means of the line to which the instrument is attached. For obtaining samples of the bottom the sounding-lead may have a cup-shaped vessel, with self-closing cover, attached to it, or a deep- sea dredge may be employed. See Deep-Sea Ex- PLOBATioN; Dredge. The great bulk of the hydrographie surveying done by commercial nations is performed in the waters contiguous to the coast-lines, since it is here the shoals, reefs, currents, etc., whose loca- tion is of concern to the mariners, are chiefly found. Deep-sea surveying is more purely of scientific interest only, tiie principal direct com- mercial puri)0se of such deep-water surveys being the location of submarine telegraph cables. Turning now to the sources for gathering h_v- drographic data outside of the surveys conducted directly by the Hydrographie Office, reference has already been made to the surveys performed in locating submarine cable routes. The various cable companies have ships continuously at work making such surveys or repairing and relaying old cables, and the numerous data collected by these vessels are forwarded to the Hydrographie Office for record and compilation. In the United States the Hydrographie Office also has the aid of the work done by the Coast and Geodetic Survey in surveying and mapping the coast-line, by the Engineer Corps L'nited States Army in improv- ing harbors and rivers, and by the United States Lighthouse Board in establishing lightliouses, buoys, beacons, etc. In addition the United States Commission of Fish and Fisheries collects considerable hydrograi)hic information in the course of its work w hieh is of value to the hydrog- rapher. It is also the accepted duty of every master of a vessel to rejxjrt to the proper authori- ties every fact of importance to mariners which he may observe in his voyages. Finally, there are the various expeditions which are organized from time to time, sometimes by the Govern- ment and sometimes by scientific bodies, to study the physical geograpliy of the ocean. The first, and in some respects the most important, of these expeditions was that of the Challenger, sent out by England in 1872, and a reference to the brief description of its work (see Chaliexgeb Expedition) will serve to explain the nature and purpose of subsequent similar expeditions. The hydrographie work so far mentioned has referred exclusively to navigable waters. In re- cent years constantly increasing attention has been devoted to the conservation and utilization of surface waters for irrigation in arid regions, for water-power and water-supply, and for vari- ous other utilitarian purposes. In the United States there is a special department of the Geo- logical Survey devoted to the collection and puh- lication of hydrographie data of this sort. Simi- lar work is done by other governments, particu- larly that of England in India. As in the case of navigable waters, here also much information is obtained from the work done in developing private enterprises, such as water-power and water-works tmdertakings. Recording and PrnLiCATiox of HTDROGRArnic Dat.<. The work of placing on record and mak- ing public the facts collected by the Hydrographie