PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 
937 
spring. However, I must here caution the reader that at this time of year, when 
the propulsive force for gradient currents is derived mostly from the deep strata of 
Fig. 188.— Dynamic gradient at the surface of the gulf for February to March, 1920, referred to the Eastern Channel as the 
base station. The dynamic heights are given for every dynamic centimeter. For further explanation see p. 937 
water, the probable error introduced into the calculations by the necessity for assum- 
ing an arbitrary correction for the differences in depth between pairs of adjacent 
stations (p. 934) is relatively greater than for late spring or summer, when the surface 
stratum is moving more rapidly than the underlying water. Consequently, the 
contour lines on the early spring chart (fig. 188) and the dynamic gradients which 
