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. ^^ 



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 

 aflfect 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 63^ miles. Dominant drifts of greater or less magnitude also result from tidal 

 measiu-ements 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 vphole 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 plottiag 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 



*3It is convenient to use a position plotting sheet, such as can be had from any dealer in navigational supplies. 



