While the boat has been running the lines of soundings, the water has not remained at the same level owing to the rise or fall of the tide, and, therefore, the tides must be observed by means of a staff or self-registering tide gauge while the sounding is in progress, so that the depths can be reduced to a common plane of reference. Each sounding must therefore' be corrected for the stage of the tide at the time it was taken. The plane of reference usually adopted is that of 'mean low water,* which is, roughly speaking, the mean reading of all the low waters observed on the tide staff for as long a period as practicable, hut usually not less than one lunar month. The reading of mean low water on the tide staff being known and the observations of the height of the tide having been made at sufficiently frequent in tervals while the soundings were in progress, the tidal corrections may be readily applied to the measured soundings. In order to preserve the plane of reference permanent tidal bench marks are established on shore and the plane of reference is referred to them by leveling. In deep-sea sounding, offshore it is not necessary to apply the tidal reduction to the soundings.
The successive positions of the sounding boat being plotted; and the number and time in tervals of the soundings being shown in the record, intermediate soundings are spaced ac cordingly between the positions. The result is practically the same as though the boat's posi tion had been instrumentally ascertained at each sounding. Up to depths of about 20 fathoms the soundings can readily be made by the hand line; for greater depths machine sounding appliances are used. Down to about 85 fathoms, these appliances are designed to be used while the sounding vessel is under 'way; at greater depths, including deepsea work, the vessel is stopped while the sounding is made. When sounding in the deep offshore waters, piano wire is used on the drum of the sounding machine, and spherical or pear-shaped shot weighing about 60 pounds is used as a sinker and is attached to the wire by a device which automatically detaches it on reaching the bot tom. The wire is then reeled in by means of the reeling engine and a new sinker attached for the next sounding. The depth of water is measured by noting the length of wire run out, and, as a check, this is again noted in reeling in. In addition to ascertaining the depth of the ocean, a mechanical device is also attached to the end of the sounding wire by means of which a specimen of the materials of the ocean bottom is secured and brought up in deep-sea sounding. While the vessel remains in sight of land the method of determining _position in naming the sounding line is similar to that explained in relation to sounding boats and launches in inshore and harbor hydrography, but when out of sight of land the position is ob tained by radio-direction plotting or by astro nomical observations and dead reckoning.
Hydrographic surveys with the lead line, while developing the slopes of the bottom, do not with certainty reveal all of the hidden dan gers to navigation that lie in the pathway of ships. In regions like the coast of Maine and in Alaska, where there are isolated rocks and ledges on the bottom, or in the tronical regions fringed with coral reefs and dotted with coral heads, the work must be supplemented by ex amination with special appliances. Owing to the extension of commerce and its increasing demands, bringing into use ever larger and deeper draft vessel ,s much special examination is now required. To meet these requirements
and in order to be certain that an area is free of obstructions to navigation to a specified depth, the water area is swept over by an ap pliance known as a wire drag, which is essen tially a large wire rope (lengths up to 12,000 feet have been used), supported at intervals by specially designed buoys so arranged that the wire can be maintained at any required depth irrespective of the changes in the stage of the tide. This apparatus is towed through the water by two or three launches, and, if no ob struction is encountered, it is certain that no dangers exist at a less depth of water than that at which the drag was set. One of the best features of the wire drag is the rapidity with which an area can be examined. The length of drag now commonly used is 3,000 feet or over, except when used in channels of less width than this. The average speed of the drag through the water is about a mile and a half per hour, and the area that can be covered in a day depends largely upon local conditions, such as natural or artificial obstructions. Great accuracy is required of the hydrographer, even where the water is so deep that there is no pos sible danger of the largest ship afloat touching bottom, because the hydrography shown on a mariner's chart has a twofold object: First, to point out to the navigator the invisible dangers which he must avoid; and second, to reveal the configuration of the bottom and also its physical characteristics so truly that by the use of his sounding appliances he may fix his posi-1 Hon in relation to those dangers, or, when off shore or the coast line is invisible, determine his distance from land.
Marine hydrography also embraces the ob servation and reduction of the tides and the tidal currents. Tidal phenomena present them selves to the observer under two aspects — as an alternate rising and falling of the surface of the sea and as a recurrent inflow and out flow of the waters. In common and general usage, the name tide is applied both to the vertical and the horizontal movements, but, in the strict sense, its meaning is confined to the changes of elevation, while the recurrent streams are properly distinguished as tidal cur rents. White the rise or fall of the tide in a given period may be practically uniform amount and nearly simultaneous at widely separated points within an extensive area, the currents which accompany the change will vary widely within the same region, being modified by every outline of the shore and by every ir regularity of the bottom. Moreover, the time at which the tidal current changes its direc tion, as from flood to ebb, or the reverse, does not generally coincide with the time at which the tide begins to fall or to rise.
The basis of fact derived. from the observa tion of the tides in hydrographic surveying provides for the prediction of their occurrence, and, accordingly, on account of the vital import ance of tidal knowledge in navigation, the United States issues an annual volume of tide tables in which the predicted times and heights of every high and low tide throughout the year are stated for 70 of the principal parts of the world, along with provision, by means of tidal differences, for affording like information in relation to more than 3,000 other ports in all parts of the world. See NAVIGATION.