PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 931 
nating application of mathematical methods first offered a practical and easy method 
of solution. 
Since that time European, and especially the Scandinavian, oceanographers 
have devoted much attention to the dynamic calculation of ocean currents, with 
such success that great advances in our knowledge of oceanic circulation are to be 
expected. Sandstrom (1919) has also studied the dynamics of Canadian Atlantic 
waters; Wiist (1924) of the straits of Florida and neighboring parts of the Atlantic; 
and Smith (1926, 1927) of the “Labrador” and “Gulf Stream” currents around the 
Grand Banks. 
The simplest and most graphic method of learning the directions followed by 
the dynamic circulation in any sea area is by a horizontal projection showing (by 
contour lines) the regional variations in the thickness of the column of water included 
between the surface of the sea and the level at which some given pressure, equal 
for the whole area, is reached. 
If the specific gravity 89 of the water is regionally uniform over the whole area, 
the depth of the layer so bounded will equally be uniform, and there will be no 
dynamic flow from any one part of the picture to any other; but if the weight of an 
equal thickness of water be greater (i. e., its specific gravity higher) at one locality 
than at another, a lesser thickness will produce a given pressure at the heavy station 
rather than at the light, and such a flow will tend to develop. 
Consequently, calculation of the height of the column of water necessary to exert 
a given pressure for any two stations will give the dynamic tendency existing between 
them in the stratum included in the calculation; and if the survey can be extended 
to include a number of stations, scattered netlike over any part of the sea, we arrive 
at the dynamic gradients for the whole area. 
This calculation is based on the principal that the pressure exerted by a column 
of water of unit area is the product of three arguments — its height, its specific gravity, 
and the acceleration of gravity; and if the first and the last of these be combined 
into dynamic units of measurements, as explained below (p. 932), pressure may be 
stated still more simply as equal to the height of the column (in dynamic units), 
multiplied by its specific gravity. Or, conversely, the height of the column (in 
dynamic units) will equal the pressure it exerts, multiplied by the reciprocal of the 
specific gravity of the water, namely, by its specific volume. 
For example, if the specific gravity of a given column of water be 1.026, and it 
be desired to find the height or depth (in dynamic units) necessary to exert 50 units 
of pressure, we have: Specific volume 0.97466 X 50 = 48.73300 dynamic units of depth. 
If at a neighboring station the specific gravity is only 1.022, 48.92350 units of depth 
will be requisite to effect this same pressure, so that there will be a dynamic slope 
between the two stations of 0.2 dynamic units of height (or depth). 
m A brief definition of the much-abused term “density” as employed to express the specific gravity of sea water 
follows: 
In hydrodynamic calculation what is important is the specific gravity that the water in question actually possessed at its 
temperature at the time and under the pressure to which it was actually subjected— i. e., in situ; not that which it might have 
possessed at any other temperature or depth. 
The specific gravity of sea water differs from that of distilled water only in the second and subsequent decimal places. To 
avoid the use of such long decimal fractions it is usual to subtract 1 and to multiply by 1,000, substituting the term “density ” for 
“specific gravity.” For example, the density of sea water of a specific gravity of 1.025 is stated as 25.00. 
Specific volume (merely the reciprocal of density)! s tha more convenient value to use in numerical calculations 
