114 HYDROGRAPHY HYDRO IDS sary, 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 sound- ings are taken at intervals as nearly regular as possible. These soundings, together with the time at which they are taken and the horizon- tal 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, re- duced 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 hydro- graphy the position of the vessel is determined from time to time by careful and numerous ob- servations of the sun and stars, and by dead reck- oning. The line used has recently been success- fully replaced by a wire, and the lead or shot at the end of it is so arranged as to bo detached on striking the bottom. An instrument called an indicator is attached to the sounding line, which, by means of revolving disks put in mo- tion 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 ac- curacy 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 Tro wbridge and Brooke, the lat- ter gentleman's having given thus far the best results. 3. Hydrographic Surveys. The pro- cess 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 geo- detic survey ; and upon it he also constructs his lines of soundings. Usually two, and sometimes three officers are employed in each boat in run- ning the lines, the advantage of this arrange- ment 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 in- tervals 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 survey- ing parties, from the chief to the leadsman, are specially trained for the work, and the re- sulting accuracy of such a survey is corre- spondingly great. Physical hydrography inves- tigates 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 endea- vors to provide remedies for the changes which injure a harbor, and to suggest means for im- proving the channels. This branch of the sci- ence has of late years attained to great impor- tance both in Europe and the United States, and the researches of those who have devo- ted themselves to its study have resulted in in- calculable benefits to commerce. (See COAST SURVEY, vol. iv., p. 761.) In regard to cur- rents, and other hydrographic details, see AT- LANTIC OCEAN, and DREDOING (DEEP-SEA). HYDROIDS, the lowest order of acalephs or jelly fishes, including, according to Agassiz, two distinct forms, one resembling polyps, the other like the jelly fishes, there being every possible gradation between the two. It is in this order that the phenomena of alternate generation have been specially studied by Sara and others. (See JELLY FISH.) There are many plant-like forms which give a mossy cover- ering to seaweeds and stones, producing buds, developing in some cases into free medusa?, and in others remaining attached to the parent stalk, both discharging ova which swim off by ciliary processes to establish new fixed hydroid communities. In the tubularians the hydroid is pedunculated, and the bell-shaped medusse are either free as in coryne or persistent as in tubularia. In the sertularians the hydroid is always pedunculated and attached, protected by a horny sheath, forming a cup around the head, with free rnedusaa as in campanularia, or free generative buds as in sertularia; their medusas are flatter than in tubularians. The siphonophora, like the Portuguese man-of-war, are also hydroid communities. The common green hydra of fresh water (Jiydra viridis) is easily seen by the naked eye ; the body is a cylindrical tube, with thread cells, and a green coloring matter believed to be the same as the chlorophyl of plants ; at the base is a disk -like sucker for its attachment to foreign bodies ; it is usually suspended, head downward, from some aquatic plant, changing its position at will. The mouth is at the opposite end, sur- rounded by 5 to 15 very contractile tentacles, armed with lasso cells, hollow, and communi- cating with the general and stomachal cavity of the body ; by these they obtain their food, which consists of minute aquatic animals. There are no internal organs of any kind, and they are therefore very little higher than the protozoa. They resist without destruc-
