PHYSICAL OCEANOGRAPHY OF THE GULP OF MAINE 969 
prevailing winds (and the strongest winds), between west and north, drive the 
surface water offshore to the southward. 
By this reasoning wind currents go far to explain the very interesting fact that 
in April the freshening effect of the spring freshets is so much more evident (in low- 
ered salinity at the surface) along the coast sector west and south of the Kennebec 
than it is off Penobscot Bay (fig. 101 ). The discharges from the former, from the Saco, 
and from the Merrimac, driven southward by the prevailing northwesterly winds of 
March and April, parallel the trend of the coast and so preserve the identity of the 
coastwise belt of low salinity. Off Penobscot Bay, however, the more or less active 
upwelling that must follow this same southerly drift off this west-east coast line, 
combined with tidal stirring, tends to prevent the development of so fresh a band 
next the land, but at the same time to carry the least saline water farther out from 
the land. The distribution of salinity at the surface for March and April, 1920 
(figs. 91 and 101), is of this sort. 
It is probable that the development of a tail of very low salinity from the St. 
John River southward across the Bay of Fundy in April (p. 808) similarly reflects a 
southerly set caused by the northwest winds, which often blow strong there during 
the first month of spring, though their average direction veers through west to 
southwest during April. 
The pool of low-surface salinity spreading out to the southwest from Nova 
Scotia, which appears on the surface chart for March, 1920 (p. 703; fig. 91), like- 
wise finds plausible explanation as a wind-driven drift out from the bays south of 
Yarmouth, where northerly winds prevail in February (p. 966). 
The effects of the winter winds are more puzzling in the eastern side of the basin 
of the gulf, where prevailing west-north winds tend to produce a southeasterly or 
southerly drift at the surface, but where the evidence of salinity and temperature 
points to a movement in just the opposite direction — i. e., northerly toward the Bay 
of Fundy in winter as well as in summer (p. 910). 
It is evident here that although strong northerly winds may and no doubt do 
temporarily drive the surface water southward, the general dominant drift is caused 
not by the wind but by other forces (p. 976) strong enough to overcome the wind 
effect in the long run. Consideration of the depth to which wind currents may be 
set in motion corroborates this conclusion, because the frictional depth of the average 
winter wind of about 4, on the Beaufort scale, is theoretically only about 67 meters. 
Actually, the water of the eastern side of the gulf not being homogeneous, the depth 
of the wind current will be something less than this — perhaps 50 meters with the 
state of stability prevailing in winter. The thickness of the stratum which the wind 
can set in motion at an appreciable rate is still less. 
According to the long series of observations on wind and current that have been 
carried out by the United States Coast and Geodetic Survey, the velocity of the wind 
current is 1.5 to 2 per cent tnat of the wind — say, about 0.4 knot, with a wind of 4 
(Beaufort scale, 20 nautical miles per hour). Smith’s table of theoretic velocities 
(Smith, 1926, p. 46, Table 8), applied to a current of this strength with assumed 
frictional depth of 50 meters, gives a residual current of only 0.2 knot at a depth of 
10 meters, about 0.15 knot at 20 meters, and 0.07 knot at 30 meters. Theoretically 
(in a free ocean), in the example just stated the current at 10 meters should set 36° 
