HYDROGRAPHY. 515 



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 acci- 

 dent, often from disaster. Such dangers are found from time to time 

 in the most fi-equented 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 

 cliart. 



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 dis- 

 charging waters, and the sediment and debris 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 frequent- 

 ed harbors of this nature require reexamination 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 Con- 

 gress 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 con- 

 structing and publishing charts, sailing directions, and all hydro- 

 graphic 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 reconnoissances and running surveys. 



