THE 



POPULAR SCIENCE 

 MONTHLY. 



MARCH, 1876. 



HYDROGRAPHY. 



HYDROGRAPHY (from the two Greek words, vdop, vjater, and 

 Ypd(p(x), description, is the important branch of physical science 

 and descriptive geography which has for its object the graphical rep- 

 resentation of the waters of our globe and their shores, with all their 

 proj^erties bearing upon navigation. 



Their exploration to this end, their description by means of charts 

 and directions for the use of the navigator, as also the generalization 

 of the local data in order to ascertain the laws governing the physical 

 phenomena upon which navigation depends, the winds, currents, 

 weather, tides, terrestrial magnetism, etc., is the responsible and ardu- 

 ous task of the nautical surveyor and hydrographer. 



The most essential requirement for navigation is charts, general 

 charts of entire oceans, or parts of them and their shores, compiled 

 by the hydrographer from existing data, and special charts of smaller 

 areas, of harbors, roadsteads, etc., prepared from special surveys. 

 The earliest sea-charts date from the middle of the fourteenth cen- 

 tury. They were necessarily rude and imperfect, the earth's shape 

 and dimensions being then unknown, the log for measuring nautical 

 miles not in use, and the means for ascertaining astronomical posi- 

 tions very imperfect. 



The discovery of America and the Cape of Good Hope, together 

 with the reformation in astronomy by Copernicus and Galileo, insti- 

 tuted a new era in geographical knowledge ; the earlier charts of this 

 period, however, were still grossly inaccurate, especially as regards 

 geographical positions, and many of the isolated islands of the Pa- 

 cific Ocean, seen and described by the early Spanish voyagers, have 

 been searched for at later periods in vain, until islands in positions 

 differing hundreds of miles from those given to them, but answering 

 their description completely, have been adopted for them ; many of 

 VOL. Tin. 33 



