936 BULLETIN OP THE BUREAU OF FISHERIES 



Once the dynamic gradient is established between any two stations, the corre- 

 sponding velocity of the water at the one, relative to the other, is calculable by a 

 simple formula, described by Smith (1926, p. 31), who also makes clear the correction 

 necessary to learn the true velocity if the profile in question does not cut the current 

 at a right angle. An alternative method of calculating the velocity, often employed, 

 is described fully by Sandstrom (1919); and the Fish Hawk data for Massachusetts 

 Bay (June, 1925) have been treated in this w-ay by R. Parmenter (p. 949; figs. 198 

 and 199), as an illustration. 



DYNAMIC CONTOURS AND GRADIENT CURRENTS 



FEBRUARY AND MARCH 



At the end of the winter and during the first days of spring, when the general 

 equalization of temperature and of salinity (already discussed) makes the upper 40 

 meters extremely uniform, regionally as well as vertically (pp. 522, 703), over the 

 whole gulf, the distribution of density at the surface would suggest a very quiescent 

 state. Thus, the surface chart for February and March, 1920 (fig. 187), shows a 

 maximum regional variation of only about 0.4 units over the whole basin, with the 

 central part of the latter virtually uniform (at 25.8 to 25.9) from station to station. 



Only the immediate offing of the Kennebec River was then appreciably less 

 dense (about 25) at the surface, the Eastern Channel and the region off its mouth 

 slightly more so (about 26 to 26.1); and the whole western and central part of the 

 gulf, with the coastal belt along Nova Scotia, was then equally uniform at 40 meters, 

 though with slightly higher values (26.3 to 26.5) along the eastern side of the basin 

 and through the Eastern Channel. 



It is clear that with the water so nearly homogeneous horizontally there is very 

 little dynamic tendency toward any general system of gradient currents in the upper 

 stratum of the gulf at that season, except that the freshening of the surface by the 

 increasing flow from the Kennebec foreshadows the development of a drift westward 

 along the coast — a tendency, however, still confined to so thin a surface stratum 

 that it did not yet govern. 



Neither does the state of the water at the surface suggest a general dynamic tend- 

 ency at that season toward a drift from the east into the gulf past Cape Sable, or 

 vice versa, in the surface stratum, the density of the upper 40 meters being com- 

 paratively uniform (in horizontal projection) from the cape out to Browns Banlc for 

 early March. This corroborates the evidence of salinity and temperature that the 

 Nova Scotian current did not flood westward past the cape in the spring of 1920 

 until later than sometimes happens (p. 832). However, when the density of the deep 

 strata is taken into account it becomes obvious that the hydrostatic forces set in 

 operation by the banking up of the heaviest water against the eastern slope of the 

 gulf (p. 849, fig. 172) must tend to cause a cyclonal or anticlockwise movement of 

 the deeper mid strata, carrying with it, dfe an overlying blanket, the surface stratum, 

 itself so nearly quiescent. 



The dynamic chart for February and March, 1920 (fig. 188), gives an indication 

 of the stream lines to be expected at the surface under the conditions of temperature 

 and salinity then existing, which may be taken as typical of the first two weeks of 



