930 
BULLETIN OF THE BUREAU OF FISHERIES 
dynamic overturning ever occurs in the open gulf, unless on a small scale and con- 
fined to a very thin superficial stratum. This statement is based on the fact that 
the density has been slightly lowest at the surface at all our winter stations, when 
compression is allowed for, though without this factor the surface stratum would 
often appear heaviest. It is true that the stability of the water is virtually nil in 
winter; but tidal stirring and the stirring effect of the wind are everywhere so active 
during the cold months that they more than keep pace with the chilling of the sur- 
face by constantly bringing up new water from below to take the place of the 
surface layer as the latter chills and before it is heavy enough to sink. 
The thermal effect of mechanical mixing is essentially the same as that of 
dynamic overturning, however— i. e., to bring the whole column of water within the 
chilling influence of the low air temperatures. It is possible that dynamic over- 
turning does occur locally in the coastal zone, but it has not actually been recorded 
there. 
Vertical dynamic circulation of another sort was observed in Massachusetts 
Bay in February, 1925, where water, chilled at the surface close to the land, was 
moving offshore on the bottom, and with surface water from offshore moving in 
above it to take its place, as described above (p. 659). A more detailed survey of 
the temperature of the coastal belt in winter may show that circulation of this sort 
is more widespread than appears from observations taken so far. 
DYNAMIC EVIDENCES OF CIRCULATION 
CONSTRUCTION OF DYNAMIC CHARTS 
Given a difference of pressure between any two stations in the sea, a current 
will result as surely as water will flow out through a dam when the sluice gate is 
opened, unless opposed by a stronger counterforce or an unpassable barrier. Even 
a preliminary examination of the dynamics of the gulf (and no more is attempted 
here) may be expected greatly to amplify such knowledge of its dominant circulation 
as has been gained from the more direct lines of evidence discussed in the preceding 
chapters. 
The method of attack chosen here is that of the dynamic-contour chart, 
widely employed by European oceanographers and recently described by Smith 
(1926). For the sake of the nontechnical reader, an explanation of the principles 
involved in its construction and its interpretation are attempted here in the 
simplest possible language. 88 
In the sea, gravity, acting always directly downward, will set the water in 
motion if its surface slopes at all; and even if the surface of the water be level, 
currents will be caused if its specific gravity is greater at one place than at another, 
because the pressure exerted by the water at a given depth must then vary corre- 
spondingly, and the plane at which the pressure is uniform must be oblique to the 
pull of gravity. All this is embodied in the old adage, “water seeks its own level.” 
Although the physical principles that govern the gradient currents in the sea 
are simple, calculation of the drifts that will actually result from any given distribu- 
tion of specific gravity is so complex that Bjerknes’s (1898, 1910, and 1911) illumi- 
88 See also Sandstrom (1919) for a simple explanation of hydrodynamic principles. 
