AHEEICAN ASSOCIATION FOE THE ADVANCEMENT OE SCIENCE. 67 



safe inferences there. The element of magnetic dip, though, less important practi- 

 cally than that of declination, is of value in navigation in certain latitudes, and from 

 its connection, through Gauss' investigations, with the declination and intensity, 

 assumes a high degree of importance. While the declination observations on this 

 coast go back to the seventeenth century, the dip has only been accurately observed 

 for 23 years ; for the earliest observations made in 1782 "were, from the imperfection 

 of the instruments, of little value. During this period the dip has decreased, reached 

 a minimum, and begun again to increase, so that it has been a highly interesting 

 period for observation. The lines of equal dip have been deduced by Professor 

 Loomis, from the observations which he had accumulated before the date of his pa- 

 per. The present memoir includes additional results, and discusses 161 observations 

 made at the different stations between Toronto on the north, and Baltimore on the 

 south. The average probable error of the result at any one station is about one 

 minute and six-tenths of dip, and the time of minimum dip is ascertained to be about 

 two years and seven-tenths. ' This time was the year 1843, or rather the close of 1842 

 (1842-7). Mr. Schott points out why these results do not agree with Professor 

 Hansteen's, who had not observations enough to determine the epoch of minimum 

 dip with accuracy. Observations on the "Western coast confirm these results for the 

 Eastern. 



ON THE CAUSE OF THE INCREASE OF SAN0T HOOK. BY PROF. BACHE. 



It is well known, as one of the developments of the Survey, that the Hook is 

 gradually increasing, growing to the northward into the main ship channel. At a 

 spot north of the Hook, where there were forty feet of water when Captain Gedney 

 made his survey, in less than ten years it was nearly bare. The importance of 

 determining the cause of this increase, as leading to the means of controlling it 

 cannot be over estimated. The Commissioners on Harbor Encroachments had 

 early attended to the matter and requested that the necessary observations for its 

 investigation should be made. These were under the immediate direction of Prof- 

 Bache, the observations having been made by Henry Mitchell, one of the 

 sub-assistants in the Coast Survey, with all desirable zeal and ability. 



Various causes had been assigned for this growth from the action of the waves and 

 the winds, sometimes on the outer side and sometimes on the inside of the Hook- 

 The effect of the opening and closing of Shrewsbury inlet had also been insisted 

 upon. 



To examine these and other probable causes laborious observations of tides and 

 currents had been made in the vicinity of stations which Prof. Bache showed upon 

 the map. Careful measurements of the low water line had also been made in con- 

 nection with these observations, and with others of the force and direction of the 

 winds. Objects easily distinguished from the sand, and of various specific gravitie s 

 and shapes, had been deposited near the shore of the Hook to determine the power 

 and direction of transportation of matter along the shores of the Hook. The results 

 of these observations have not yet been worked out in all their detail, but the con- 

 clusions from them are perfectly safe, and are of the highest importance. Itturns 

 out that this growth of the Hook is not an accidental phenomenon, but goes on 

 regularly and according to determinable laws. The amount of increase depends 

 upon variable causes, but the general fact is that it increases year by year, and the 

 cause of this is a remarkable northwardly current, the amount and duration of which 

 these observations assign along both shores of the Hook, the outer one extending 

 across the whole breadth of False Hook channel, with varying velocity, and the one 



