PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 
859 
the most convenient and yields approximations close enough for most purposes: Lay 
down a meridian, marking it N. and S. Then simply plot, to scale, the average 
distance and direction of the current for each successive hour, as successive lines, 
giving to each the correct compass bearing, commencing with high water as the 
starting point. Then the distance by which the location reached at one high water 
fails to coincide with the preceding high water, measured by the same scale, gives 
an approximation to the distance covered by the dominant set in one tidal day. 
The angle between the line connecting the two and the meridian first laid down 
gives the approximate direction. 53 
It is obvious that the smaller and more frequent the time intervals for which 
the mean velocity and direction are determined by the current meter, the closer will be 
the approximation yielded by this method of graphic summation, or by any other. 
The work of the two governmental surveys just mentioned (of Canada and of 
the United States) has been directed primarily to the study of the tides as these 
affect navigation. Mitchell (1881), however, showed that resolution of the periodic 
observations at stations in the South Channel, on Georges Bank, and in the Eastern 
and Northern Channels demonstrated a dominant or nontidal drift at every station, 
in some cases of considerable velocity. A nontidal drift has also been published 
for many stations off Cape Cod and in the region of Nantucket Shoals (United 
States Coast and Geodetic Survey, 1912, chart to face p. 9), as well as for the vicin- 
ity of Cashes Ledge (Harris, 1907), long before the general importance of these 
drifts in the general circulation of the gulf was appreciated. 
Dawson (1905, p. 16), on the other hand, believed that the currents in the east- 
ern side of the gulf were strictly tidal, showing no “ general movement of the water 
in any one direction in this region which is at all well marked.” Mavor (1922), 
however, on submitting Dawson’s current tables to the method of graphic summa- 
tion described above, found that a dominant drift was demonstrable at every station, 
varying in “distance made good” for a single tidal period from about 1 mile to 
about 6}/2 miles. Dominant drifts of greater or less magnitude also result from tidal 
measurements taken at Portland and Boston lightships by the United States Coast 
and Geodetic Survey and at our Albatross station off Gloucester. The number of 
current stations is now so considerable that the presence of some such set is certainly 
characteristic of the parts of the gulf which they cover. 
Some resultant drift in one direction or another is, in fact, to be expected any- 
where in the open sea, set in motion by the temporary effects of the winds alone, 
if from no other cause. Whether or not such drifts as are revealed by measurements 
of the tidal currents can be interpreted as evidence of a dominant movement of the 
water as a whole depends, therefore, on their relative constancy at given stations and 
on whether they are consistent in direction, one with another, over considerable 
areas. 
This last criterion can be tested most readily by plotting on a general chart of 
the area the dominant drifts calculated for the various stations. 
The current arrows on such a chart for the Gulf of Maine (fig. 173) show this 
requirement met to a degree somewhat surprising when we remember that the obser- 
vations were scattered through a long series of years and that the “sets” at the 
t3 It is convenient to use a position plotting sheet, such as can be had from any dealer in navigational supplies. 
