Page:Popular Science Monthly Volume 8.djvu/533

Rh the soundings, by which the profile of the bottom is represented on the chart.

Sudden elevations, shoals, and especially submerged rocks, the great dangers to navigation, sometimes escape the lead as well as the eye, even in the most careful survey, and are only discovered by accident, often from disaster. Such dangers are found from time to time in the most frequented harbors, which have been surveyed with the greatest care. While the land, with the present means, can be laid down absolutely correct, the hydrographic surveyor can never be certain that he has thus represented the most essential portion of his chart.

The hydrographic features of coasts, not rock-bound, are subject to changes, gradual by the action of the sea, and sudden by natural phenomena, as great gales, etc.; volcanic activity also affects at times the rock-bound coasts. The mouths of rivers and the embouchures of inland waters are especially subject to changes by the wash of the discharging waters, and the sediment and débris carried along by them, which mostly accumulate on the bars, and are shifted to and fro by the force of the sea before they settle firmly; the depth of water in the channels, and even the course of the latter, does not remain the same for any great length of time, and some bars change with every shift of the wind. The surveys of such localities will only hold good in their general features; in the shore-lines and in the landmarks by which a vessel may approach and feel her way in; the more frequented harbors of this nature require reëxamination from time to time.

Several nations have provided for a trigonometrical survey of their coasts only, in advance of geodetical operations embracing their entire domain.

The United States Coast Survey was first organized by act of Congress in 1807, which provided for surveying the coasts of the United States, but the first labors in this field did not commence until 1817, and were shortly after interrupted; in 1832 they were resumed, and have since been carried on, with energy and but little interruption, to the present date.

The United States Hydrographic Office, for the purpose of constructing and publishing charts, sailing directions, and all hydrographic information relating to the coasts and waters outside of the boundaries of the United States, for the use of its marine, both naval and commercial, and for directing the examination and survey of the channels of commerce in foreign waters, was established under the Navy Department in 1866.

Connected trigonometrical surveys have also been instituted for the waters of the more important of the European colonies, especially in the West and East India waters and in Australia, but for far the greater part of the navigated portions of the globe the navigator will for a long time have nothing but reconnoissancesreconnaissances [sic]and running surveys,