PHYSICAL OCEANOGEAPHY OF THE GULF OF MAINE 857 



DOMINANT OR NONTIDAL DRIFT 



In the preceding summary of the tidal currents, directions and velocities are 

 given for the flood and ebb at their strength. In some localities the direction con- 

 tinues vu-tually constant throughout ebb or flood, as the case may be. In most 

 parts of the gulf, however, the current is to a greater or less extent a veering one, 

 and there is some difference in velocity between flood and ebb. The resultant of 

 movement by which any particle of water would fail to return at the end of any 

 given tidal period (averaging 12 hours and 25 minutes) to the position from which 

 it started its journey, is the dominant drift. The name "nontidal" is commonly 

 used for this; the other appellation just given is preferable, however, there being 

 some evidence that the dominant drift which we have been able to demonstrate for 

 the Gulf of Maine has its source in the tidal currents. 



On the high seas, where tidal cm-rents are weak and the dominant drifts are often 

 stronger, the ocean currents, as we now know them, have been charted chiefly by diges- 

 tion of the drifts reported in the log books of passing ships. This source of informa- 

 tion has failed to demonstrate any dominant set (as distinguished from tidal currents) 

 in the Gulf of Maine, as might be expected where the tides are so strong and the 

 resultant movement, if any, comparatively so weak. 



MEASUREMENTS OF CURRENTS 



A considerable number of measui'ements of the tidal currents have been made 

 in the Gulf of Maine by the United States Coast and Geodetic Survey at the follow- 

 ing localities: Portland lightship off Cape Elizabeth, near Cashes Ledge, three sta- 

 tions between Cape Ann and Cape Cod at the mouth of Massachusetts Bay, Boston 

 hghtship off Cape Cod, many stations at the mouth of Nantucket Sound and in the 

 region of Nantucket Shoals, Nantucket lightship, and at a series of stations situated 

 along the southern rim of the gulf from the South Channel to the ofRng of Cape 

 Sable. 



The Tidal Survey of Canada, under Doctor Dawson's direction, carried out an 

 extended survey of the tidal currents at 19 stations distributed around the Nova 

 Scotian coast from the offing of Shelburne to the Bay of Fundy, and within the 

 latter, in the years 1904 and 1907 (Dawson, 1905 and 1908). 



One current station also was occupied off Gloucester by the Albatross in March, 

 1920 (station 20051); and measurements of the velocity and direction of flood or ebb 

 were made by the Gramjpus in the summer of 1912 at several locahties in the western 

 side of the gulf. 



Thus, the western, southern, and eastern side? of the gulf are so well covered 

 that these measurements could hardly fail to reveal the dominant set (if there be 

 any) for that part of its periphery; but no systematic study has yet been made of 

 the tidal currents along the eastern coast of Maine between Portland and the 

 entrance to the Bay of Fundy. 



Before proceeding to analyze these data we may first consider briefly what sort 

 of information they may be expected to yield. 



