9 



If D in Figure 3 (6) is the distance in dynamic decimeters between 

 two points in the sea, and h is the unit vertical distance in common 

 meters, then D=g h, where g is the acceleration of gravity. In 

 (&), if we know the difference in dynamic depth units (the number of 

 dynamic decimeters) between any two points, A and B in the sea, 

 this number will be the same as the gravity potential released by a 

 unit water mass flowing from A to B. Expressed geometrically we 

 have from the figure, two points A and B between two level surfaces 

 M and N, the two latter of which are h decimeters apart. The angle 

 between line L and the planes M and N is called a 



w = L sin a ^ = h g = D. 

 where D = difference between M and N in dynamic decimeters, and 

 a is so small in all cases that sin a may be put equal to a. 



Let us, before passing on to a discussion of pressure, glance at the 

 more exact values of acceleration due to gravity at various points 

 on the earth, and also determine the corresponding values of potential 

 expressed in dynamic measure. The attractive force of the earth, g, 

 increases both with the latitude and with the depth in the sea, 

 therefore the distances between equipotential unit surfaces — i. e., 

 the dynamic decimeters — will be longer at the equator and near 

 the surface of the sea, where g is comparatively small, than at the 

 pole and near the bottom where g is comparatively large. In the 

 meter-ton-second system of units a free falling body will accelerate 

 approximately 9.8 meters in one second, therefore the dynamic deci- 

 meter, or unit of gravity potential, will be equal numerically to the 

 reciprocal of this value, or 1.02 common decimeters. Stated inversely 

 one common decimeter equals 0.98 dynamic decimeters. Simply 

 multiplying units by 10 give results in terms of ordinary meters and 

 dynamic meters, both of which are of a magnitude most convenient 

 for practical investigations in hydrodynamics. 



PRESSURE 



Pressure is defined as a force, the intensity of which msij be repre- 

 sented at any depth by the weight of a column of water of unit area 

 extended vertically upwards to the surface. The force of pressure, 

 though present at every point in the ocean, does not actually manifest 

 itself as an active agent until we extend our consideration to two 

 points and the difference of pressure arising. This statement, of 

 course, holds true more or less for all forces, but it seems worth re- 

 marking here, as sea pressure, to most people, is an effect difficult to 

 comprehend; yet a difference in pressure, such as exists when a 

 hollow sphere is submerged in the sea, immediately becomes tangible. 



Let us take, for example, the motionless ocean in wliich we con- 

 structed a system of equipotential surfaces 1 dynamic decimeter 



