Hydrography

HYDROG'RAPHY (ante). The science of hydrography received a new impulse from the celebrated rapt. James Cook, of the English navy, who introduced what is known as running surveying, but his system has been greatly improved. The com mencement of Cook's hydrographic surveys was in 1759, when he was master of the frigate Mercury, stationed at Quebec with the squadron co-operating with gen. Wolfe. He commenced a series of observations of the St. Lawrence river, which were continued until he was able to publish a chart of the river from Quebec to the Atlantic ocean. In 1763 he was sent out to survey the coast of Newfoundland, and in 1764 he received the appointment of surveyor of the coast of Newfoundland and Labrador. Wherever he was ordered he continued his hydrographie observations until the year of his death, 1779. The French had been observers of his operations, and in 1785 La Perouse was sent with two ships and a corps of scientists to visit the n.w. coast of America and to explore other parts. He made important observations there, and also on the n.e. coast of Asia. After spending two years and a half he went to Botany Bay, after which he was never heard from, except that information was obtained seven or eight years afterwards which made it probable that the ships were wrecked on a coral reef on the coast of Manic°llo. But La Perouse had sent duplicates of charts and journals up to the time of his arrival at Botany Bay. The navigating officer of the expedition, Beautemps-Beaupre, sent out to search for La Perouse 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 (1803). This, however, had been preceded by Alexander Dalrymple's essay on marine surveying, published in 1771. Beautemps-Beaupre was placed in charge. of the survey of the French coast, where he trained a number of hydrographers, the commencemant of a corps of engineers for future exploration and surveying. Most civilized nations now have their governmental hydrographic offices, and numbers of officers and men are engaged in making surveys, but England leads all other nations in this direction, having made accurate surveys of her domestic and foreign coasts, and many of those of other nations. The most important, perhaps, of all the expeditions ever sent out by Great Britain was that of the Challenger, which sailed from Sheerness on Dee. 7, 1872, and returned to Spithead on May 24, 1876, having during this time traversed a distance of more than four times the equatorial circumference of the earth, and established 369 observing stations along the course traversed. The objects, however, of this expedition extended, in many respects, beyond the observation of the configuration of the floor of the ocean or of its coast line; had regard to many botanical, zoological, and geological questions. During the first year the Atlantic was crossed six times, and a diversion made from Bermuda to Halifax and back again to make observations upon the gulf stream. The ship then went to the cape of Good Hope, and from thence southward toward the Antarctic ice barrier, and after taking observations along its margin proceeded to Melbourne, Sidney, and New Zealand. Then the western part of the great area of the Pacific was examined and the adjacent part of the Malay archipelago. On leaving this for Japan, at a point n. of New Guinea, the deepest sounding of the expedition was made, and the deepest reliable sounding, it is claimed, that has yet been made, viz., 4,475 fathoms, or more than five miles. From Japan she steered due e. as far as the meridian of the Sandwich islands; thence to that group; thence to Otaheite, as far beyond the equator; from Otaheite to cape Horn; thence to Valparaiso and back through the straits of Magellan, to the Falkland islands, to Montevideo, and thence eastward half-way across the Atlantic to complete some work partly done during the first year; thence due n. in the meridian of Madeira as far as the equator; thence n.w. at some distance from the coast of Africa, following the middle line of the north Atlantic, past the Azores, and thence home. At each of the observing stations a sounding was taken to determine the exact depth; the bottom temperature was ascertained, and a sample of bottom water obtained. Some of the bottom, from an ounce to a pound, was also brought up, and at most of the stations the temperature of the water at several dif ferent depths was taken, and also a fair sample of the bottom fauna obtained by the dredge. The direction and rate of the surface water was determined, and at times attempts were made to determine the direction and velocity of the water at different depths. In addition to this, meteorological and magnetic observations were regularly taken and recorded. The work accomplished included, among many other results, the determination of the depth and configuration of the ocean basins. But little was pre viously known of this except what had been obtained in surveying lines for telegraph cables. Facts had been observed during these cable-line surveys which were supposed to be exceptional, but the soundings of the Challenger, and those of the U. S. ship Tuscarora

and the German Gazelle, have shown them to be general. See SEA ,SOUNDING, DEEP SEA, ante. The methods practiced in hydrography vary with circumstances. When the advantages of good triangulation exist, a hydrographie chart can be constructed with great accuracy. The principles involved are the same as those in geodetic survey ing or leveling, the vertical measurements being taken with a sounding line instead of a rod, and the element of time employed to approximate horizontal distances. A boat is started at a certain point which has been determined by triangulation, and takes a course towards another point whose position is also known. The boat is then propelled with as near a uniform rate of motion as possible, and soundings are taken in succession at regular intervals during the transit, and recorded. The plotting of this line will, of course, give the depths of water al] along its course at the time it was taken. A tide gauge, which gives the state of the tide at the time, will also determine the depths of the soundings at mean low water. A number of "sounding lines," as these courses are called, having been run across the bay or harbor, or whatever sheet of water is being surveyed, and of such a number as may be thought necessary, which will depend upon the nature of the bottom, the data are obtained for the plotting of a chart. If the bot tom is known to be comparatively even, and no rocks or steep slopes have been found, and if it be convenient to do so, the lines may all be run in parallel directions. But when the bottom is quite uneven, and there are rocks or sunken vessels, these parallel lines should be crossed, as nearly at right angles as possible, by another series of parallel lines, and all should be as close together as practicable. When the shore cannot be used for triangulation a base line must be established, as well as circumstances will allow, by anchored boats whose distance apart may be computed by the time which sound is found to travel from one to the other; or, if the water be not too rough, by the average time it takes to row in both directions from one boat to the other; or a cord may be used to measure a distance too great. Sounding lines may then be run in dif ferent directions from these points of observation, between points which can be estab lished, or in directions towards prominent objects on shore, and the distance traversed estimated by such means as may be most convenient, according to the resources of the engineer. It is often necessary, especially when it is impossible to run the sounding boat with uniform motion, or where the bottom is quite uneven, or the position of rocks is to be determined, to take two observations on shore with theodolites simulta neously with any special soundings, the time being determined by a ball or flag signal, or a flash made from the boat. The angles being taken between the point designated by the signal and another established point, its locality is readily established. The hydrography of the United States is in charge of the coast survey. There is a coast survey office and a hidrographie office, the latter established in 1S60. See COAST Suit VEY, GEODESY, and TRIANGULATION, IIrDROIDS, marine animals which have been variously classified by naturalists because of the extreme difficulty of studying their natural history. Modern classifica tion makes them a sub-class, Hydroida, in the class Hydrozoa, sub—kingdom Ccelen terata, the representative of Cuvier's Eadiata. The Hydroida .possess a great deal of interest because remaining so long unrecognized in some of their phases of life. In one stage of their existence they so much resemble sea plants that for a time they escaped recognition. Patient labor, however, has at last placed them in their proper relations. These hydroids exist in compound colonies of alternate generations, one kind having the office of feeding the community, the other of reproduction. The feed ing hydroids are usually fixed, or attached to some object, and proceed from eggs of the reproductive, or medusa hydroids, the latter in turn growing from buds produced by the former. The medusa hydroids sometimes remain attached to the stein, or become free-swimming medusa The body of the nutritive hydroid is usually sup ported by a stem of variable length, but may rest immediately upon the bottom. From one individual buds appear and produce branching colonies of hundreds or thousands, often having a height of 15 or 20 inches. The reproductive hydroids are sometimes devel oped into perfect medusa before leaving the parent stem, but they usually break away before attaining their perfect state. Some buds never become much developed, and are called sporosacs. These usually remain attached, but attain sexuality and reproductive power. The free-swimming medusa often grow 9 or 10 in. in diameter, but many of them, it is said, remain very small, seldom attaining a diameter of more than an inch.

See ACALEPII,E; GENERATIONS, ALTERNATION OF; ZOOLOGY, ante.