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Prof. Bache's Report on the U, S. Coast Survey. 163 



II. The hydrographic surveys have yielded important discov- 

 eries of new and better channels, and of dangers heretofore un- 

 known. Of the former class are Gedney^s channel at the en- 

 trance of.; New York, Blake's in Delaware Bay, an entrance of 

 twenty-one feet water on Mobile bar, and a new harbor on the 

 dangerous coast of North Carolina. Of the latter, are the dis- 

 coveries of Davis' and other shoals near Nantucket, in the most 

 frequented jath of commerce, several on the southern coast, and 

 the new delineation of shOais off the coast of Maryland and off 

 y^s-pe Hatteras, differing almost entirely from the data previously 

 furnished by the best authorities. Professor Bache remarks that 

 snch results, though more striking, are not more useful, than the 

 accurate delitieation of the harbors, bays, and sea-coast, which 

 the ordinary operations are continnally furnishing to navigators. 

 He adds, that some of the differences observed between the re- 

 cent surveys and those on which former charts were founded, 

 are no doubt the -results of actual changes; but there are no 

 facts which prove that any important channel has been oblitera- 

 ted without being replaced by another, or has diminished in 

 depth. The portions of our coast which really do change will 

 be made known in the course of the survey, and the permanent 

 inarks furnished by the land work will permit re-examinations to 

 be made, wlifenever required, at small expense and by ordinary 

 nieans. The examinations relative to the entrance of Columbia 

 nver, heretofore so much.. dreaded by mariners, show that there 

 We been gcpat changes, but always a good channel, and its re- 

 cent navigation "has been without accident. 



At vadous points, observations of tides and currents have heeii 

 kept up niinutely. Those of the tides at Cat Island are under 

 discussion, as of a remarkable class, gi^^ing in general but one 

 high and one low water tide during twenty-four lunar hours. 

 ■1 hese tides are supposed to depend upon the interference of the 

 duirnal inequality (12-hour tide) wave with the semi-diurnal 

 (6-hour tide) wave, — at least their variations may be traced and 

 predicted from such a hypothesis. 



The changes of a shore line by the action of storms and 

 wmds, and the encroachments of the sea upon the land, have 

 been discussed by observers at Long Island and at Sandy Hook, 

 respectively. The former of these infers, from tradhion and 

 esammation, that deeper water is, in general, to be expected iti 

 the same places at present than formerly ; but about the same 

 ^^P^h at the same distance from the beach, as the sea probably 

 carves out the same form of bed in all ifs course of encroach- 

 ment. At Sandy Hook, where there has been a n)arked motioa 

 01 Its northern extremity lo westward, the observer finds that, 

 whenever there are natural obstructions to the wind, sand hills 

 are formed and the beach preserved. He infers the feasibility of 

 preserving and controlhng the formation of the beach, by hedges 



