296 GEOGRAPHIC WOJiK OF THE U. S. COAST SURVEY 



Its researches in physical hydrography include not only the 

 study of the tides and currents and incidentally the estahlish- 

 ment of planes of reference from which the constancy of the 

 relation hetween the ocean level and the land is to be inferred, 

 but it has studied for future comparison the movements of sandy 

 sliores, as, for instance, those of Cape Cod and of the exposed 

 islands of Nantucket and Marthas Vineyard, to discover the re- 

 lationship between the outlying shoals and the changes of the 

 shores. Here, again, precision of work alone is of any avail, for 

 correct conclusions can be drawn only after the lapse of time and 

 after a standard of comparison has been created by an accurate 

 survey. Want of space forbids the enumeration of many special 

 results, but the discovery of the value of the tidal circulation 

 through the East river as a foctor in maintaining the deitth of 

 the bar at Sandy Hook and the discovery of the underrun of the 

 Hudson and its bearing on the feasibility of obtaining a water 

 supply for the towns along that river may be mentioned as con- 

 tributions in a special field of geography. 



As properly belonging to the subject of the hydrographic sur- 

 veys, the literature of the several and successive volumes of the 

 Coast Pilots, published by the Survey, must be mentioned. The 

 Coast Pilots of Alaska, compiled b}' Davidson and later by Dall, 

 are invaluable historical records of the geography of that coast, 

 and the same ma}'' be said of the volumes covering the remainder 

 of the Pacific coast and those which describe in detail our Atlantic 

 shores. They are not intended to deal in generalities, butthe}'' 

 describe with rigid particularity geographic landmarks which are 

 to guide ships by day and by night. 



The maps of the Survey are embellished by accurate repre- 

 sentation of the topography which borders our shores. For thou- 

 sands of miles a narrow fringe of topograph}^ has been mapped 

 with minute and necessar}'- accuracy. It is based on local and 

 detailed triangulation, which in turn rests on a larger network 

 of triangles which coordinates all the surveys along the coasts. 



The introduction of precise methods for the determination of 

 latitudes and longitudes went hand in hand with all the other 

 operations of the Survey. Thus the success of Morse in the 

 spring of 1844 was followed in the autumn of the following year 

 by formal instructions given by Bache to Walker to }»repare for 

 telegraphic longitude determinations : but it was not until Oc- 

 tober 10, 1846, that the method was successfully put into prac- 

 tice by the exchange of signals between Philadelphia and Wash- 



