al 
230 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
these two bearings, the total drift being about 1.2 knot toward the 
south by east. On combining the stations, omitting the first hour of 
10074 to compensate for the advance of the tide during twenty-four 
hours, a southwesterly surface drift of 2.2 knots and a southwesterly 
bottom drift of about 1 knot results. i . 
The last two hours of the ebb are still to be accounted for; the regu- 
lar veering of the surface current suggests that it continues to swing 
toward the east and southeast, and general knowledge of similar tidal 
currents suggests a diminishing velocity. These two stations, then, 
taken together, indicate a dominant southwesterly current with a 
velocity on the surface, of two to three knots for an entire tide, 
2. €., four to six knots in twenty-four hours. Of course the validity 
of this conclusion depends on whether a combination of these two sets 
of observations, as though they had been made at one station, is 
justified, and there is no apparent objection to so doing, either in the 
contour of the bottom, the course of the shore line, or in the amplitude 
of the tide at the two stations. Nor was there anything in the weather 
conditions to suggest that the surface current was a wind drift at 
either, because Station 10074 was occupied during a calm, and after a 
calm night; Station 10072 likewise after a calm night, and in a moder- 
ate breeze. And so far as the observations go, the velocity of the tidal 
currents was apparently about the same at the two stations, béing 
about .7 knots per hour for the fifth hour of the flood at Station 
10072, .6 knot at Station 10074. The bottom currents likewise sug- 
gest a slight southwesterly drift. 
CIRCULATION OVER THE CONTINENTAL SHELF, JULY, 1913. 
Our current measurements, salinities, and densities allow a tentative 
reconstruction of the movements of the water over the continental 
shelf at the time of our visit. During the spring there must have been 
off shore surface currents opposite four main sources of fresh land 
water, 7. ¢., Long Island Sound, the Hudson River, Delaware Bay, and 
Chesapeake Bay, to produce the tongues of low salinity which we 
encountered there (Plate 2). These currents must have been at their 
height at least a month earlier, 7. ¢., at the time of the greatest 
river freshets; the Delaware current reaching its maximum after the 
middle of May, because the salinity was higher off the Bay on May 9 
(p. 188) than we found it (p. 198). The drift, as indicated by salinity, 
was easterly off the mouth of Long Island Sound, and there must have 
