BIGELOW: EXPLORATIONS IN THE GULF OF MAINE. 83 



Circulation in the Gulf of Maine. 



Circulation in the Gulf may be expected to be of three types : — 

 1, tidal, which is proverbially violent in the northeastern part of this 

 region; 2, the slower but more constant vertical or horizontal move- 

 ment of water resulting from different density gradients at different 

 regions, or from the presence of an actual ocean current, if there be one; 

 and 3, sporadic movements of the water, due to prolonged or violent 

 winds. 



Tidal currents. — A considerable number of measurements of tidal 

 movements have been made on the surface of the Gulf of Maine by the 

 U. S. Coast and Geodetic Survey, by the British Admiralty, and by 

 the Tidal Survey of Canada in charge of Dr. G. B. Dawson; but so 

 far as I can learn, the only accurate records of bottom currents are the 

 few taken on the Grampus last summer. The earlier surface records, 

 for off shore stations, are limited to the east coast of Cape Cod, Stell- 

 wagen Bank and the channels north and south of it, George's Shoal, 

 the Eastern Channel, Brown's Bank, the west coast of Nova Scotia, 

 and the Bay of Fundy; these the 1912 cruise of the Grampus extends 

 to the central part of the Gulf and to the coastal region between Cape 

 Ann and the mouth of the Penobscot. Although our records are too 

 few for a complete survey even at a given station, we always at- 

 tempted to take them as close to the mid-period of flood or ebb as 

 possible, so as to obtain the mean direction and velocity of the cur- 

 rent for a given tide; but of course, this result could be expected only 

 in regions where the current was fairly constant for the major part of 

 each tide. 



The sum of all available observations suggests that the violent surface 

 currents of the Gulf, noticed by every navigator, are purely tidal, the 

 mean flow of ebb and flood being in general about equally strong at a 

 given locality; but the mean directions of the two are not always pre- 

 cisely opposite. The general rule is that " along the whole line between 

 Nantucket shoal and Cape Sable Bank the ebb current runs south- 

 wardly. . . .the flood current northwardly. ..." (U. S. Coast Pilot), 

 and along this whole line the currents are swift (1.1 to 1.6 knots at 

 their height) . The tidal wave divides over the basin south of Jeffrey's 

 Bank and Cashe's Ledge, the flood currents west of here turning west- 

 ward toward Massachusetts Bay, and toward the coast between Cape 

 Ann and Portland. Abreast of Casco Bay (Stations 14, 40) the flood 

 flows nearly due north; but east of Jeffrey's Bank the general direction 



