962 
BULLETIN OF THE BUREAU OF FISHERIES 
surrounded by lower densities to the south, and so separated from equally heavy 
water to be expected near the Eastern Channel and through the trough of the latter, 
just as was the case in July and August, 1914. The available data thus suggest that 
the dynamic tendency toward circulation continues regularly anticlockwise from 
summer to summer in the northern and northwestern parts of the gulf, though dif- 
ferences in the location of its center of revolution and in the regional distribution 
of density off the western shore are correspondingly reflected in the stream lines. 
AUTUMN AND WINTER 
Progressive equalization of temperature taking place in the shoaler strata of the 
gulf during the autumn obliterates the pool of low density that characterizes the 
offing of Massachusetts Bay in summer. As a result, the distribution of density 
comes to conform more and more closely to that of salinity. In the midwinter of 
1920-1921 (apparently a representative season), the upper 100 meters were less dense 
around the coast than in the basin offshore, with the transition more abrupt in the 
western side than in the eastern, and the values highest in the offing of Cape Ann 
(station 10490). 
A regional inequality of this sort must cause a dynamic tendency for the 
coastal belt to drift parallel with the land anticlockwise around the gulf, much as in 
spring (p. 942), producing a northerly set along Nova Scotia, westerly along the coast 
of Maine, and southerly from the offing of Cape Elizabeth past Cape Ann to Mas- 
sachusetts Bay, relative to the underlying water mass. This latter (as represented by 
the 150-meter level) then proved nearly uniform in density horizontally (i. e., was 
nearly stationary). Unfortunately, no data are available for the southern or south- 
eastern parts of the area for midwinter. 
The progressive mixing of the water that takes place as winter advances makes 
the upper stratum more and more uniform, both horizontally and vertically, with 
respect to density as well as in temperature and salinity, until by February it becomes 
nearly homogeneous, as described above (p. 522), and the annual cycle is complete. 
WIND CURRENTS 
Seafarers have known, from the dawn of history, that the wind sets up surface 
currents often so strong that they must be taken seriously into account in navigation; 
and many a good ship has been wrecked from ignorance of the wind current. 
In the Gulf of Maine the motive effect of the wind is made most apparent to the 
oceanographer by the upwellings of colder and salter water from below, which take 
place along its western margin when the surface water is driven offshore (p. 550). 
Every fisherman along our coasts knows from first-hand experience that strong winds, 
blowing from one quarter or another, strengthen the ebb at the expense of flood — or 
vice versa, as the case may be. 
The dynamic principle according to which wind currents are produced is extremely 
simple: The wind drives the surface water before it, the motion of the latter being 
propagated to underlying strata by the internal friction of the water. Once in motion, 
the water, as Nansen (1902) and Ekman (1902) have pointed out, must be deflected 
by the effect of the earth’s rotation. Nansen’s (1902) observations on the drift of 
