948 BULLETIN OF THE BUREAU OF FISHERIES 



of April (Ice Patrol stations 2 to 3 and 21 to 23, p. 997). The regional distribution 

 was essentially the same on this profile for May 4 to 7, 1915 (fig. 196), and it is 

 because this Nova Scotian water is relatively so light that it so little affects the 

 temperature of the deep strata of the gulf. 



This overflow of water of low salinity shifts the potential depression, or low 

 (representing the center of high density), from east to west across the gulf to the 

 offing of Massachusetts Bay (figs. 194 and 195) — i. e., to the situation where the 

 surface is high in summer (p. 956). So long as the regional distribution of density 

 is of this sort (from early May in some years ; probably as early as April in others) 

 the anticlockwise vortex centers over the western arm of the basin 30 to 50 miles 

 out from the mouth of Massachusetts Bay. 



Under these conditions the surface water may be expected to drift with consid- 

 erably greater velocity from northeast to southwest around the western margin of 

 the gulf than from south to north along its eastern trough (fig. 195), though the 

 current may be equally strong next the west coast of Nova Scotia, where data for 

 May are lacking. To what extent this anticlockwise circulation involves the Bay of 

 Fundy in that month is yet to be learned, though the sudden freshening of the sur- 

 face there by the freshets from the St. John Kiver (p. 808) suggests a considerable 

 differentia] in density between the two sides of the bay as characteristic of May, 

 pointing to an outflow in its northern half. 



The data for 1915 fail to outline the longshore drift farther south than Cape 

 Ann, lacking observations close in to the cape or in Massachusetts Bay, but the very 

 low densities recorded at the mouth of the bay in May, 1920 (fig. 194), show it contin- 

 uing down past Cape Cod, consistent with the drifts of bottles set out in Massachu- 

 setts Bay m April, 1926 (p. 893). 



The dynamic gradient is so much steeper at the surface than in the deeps of the 

 gulf in May that calculations of the relative velocity would approximate the truth 

 more closely then than earlier in the spring. In 1915 the calculated velocity relative 

 to the low off Cape Ann (assumed stationary, fig. 195) was about 0.23 knot per 

 hour near Cape Elizabeth, or about 53^ nautical miles in 24 hours. Abreast of 

 Mount Desert, however, the calculated velocity was only about 0.14 knot toward 

 the west at the time. 



Unfortunately no dynamic data are available for the southeastern part of the 

 area for May, so that nothing can yet be said about the effect that the Nova Scotian 

 current may exert on the gradient currents of the Eastern Channel and vicinity. 



JUNE 



No one of our cruises affords a general dynamic picture of the gulf as a whole 

 in June, but the state of its eastern side shows that in 1915, at least (fig. 197), the 

 slackening of the Nova Scotian current from the east, coupled with the vernal 

 warming and progressive incorporation of land water in the west, caused the 

 low center of anticyclonic circulation to shift from the offing of Cape Ann to the 

 Eastern Channel by the last week of June. This seasonal return to the location it 

 occupies in March (judging from 1920) probably represents the normal progression, 

 the physical changes on which it depends being yearly events. 



