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*' 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, multiphed 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). 



••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 difEers 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 1.035 is stated as 25.00. 



Specific volume (merely the reciprocal of density) is the more convenient value to use in numerical calculations. 



