PHYSICAL OCEANOGRAPHY OP 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 virtually 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 currents 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 measurements 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 
lightship 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 offing 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 Grampus in the summer of 1912 at several localities in the western 
side of the gulf. 
Thus, the western, southern, and eastern sides 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. 
