HYDROGRAPHY. 



By FRANCIS M. GREEX, 



LlEUTENANT-COilMANDEK, U.S.N. 



Hydrographic surveys have been prosecuted during the 

 last year in nearly every part of the earth, increasing in ful- 

 ness and accuracy our knowledge of the shores and islands 

 of the various oceans, and constantly diminishing the very 

 large extent of coast-line yet unsurveyed and only imper- 

 fectly known. 



In this task American, English, French, German, Italian, 

 Spanish, and Austrian naval officers have been engaged, the 

 needs of commerce calling loudly for more exact knowledge 

 of many channels of navigation which can only be made safe 

 by systematic and exact government surveys. 



When this information is required on the shores of half- 

 civilized countries, such surveys must necessarily be made by 

 the more enlightened maritime nations ; and in such work 

 Great Britain and the United States have always been ready 

 to aid. 



The exchange of hydrographic data between the govern- 

 ments of maritime nations, constantly and systematically 

 carried on, adds a large volume every year to existing hy- 

 drographic information. So completely is this work ef- 

 fected that any alleged discovery of a rock or shoal, or a 

 change in lights, buoys, or other aid to navigation which 

 may be communicated to the hydrographic office of any mar- 

 itime nation is instantly sent to all the others, thereby in- 

 suring the spread of the knowledge all over the world with 

 the smallest possible delay. This custom has grown up so 

 quietly and gradually that the world at large knows but 

 little of a system which adds enormously to the safety of 

 navigation. 



Of these notices the United States Hydrographic Office, 

 under the superintendence of Captain S. R. Franklin, U.S.N., 

 has, during 18*78, issued 85 relating to changes in the earth's 



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