8 
A free-falling body, regardless of time or its velocity of descent, 
will be continuously accelerated at the constant rate of about 10 
meters per second. For the purpose of measuring forces in the sea 
we wish to construct a series of coordinate equipotential surfaces, 
not merely a linear distance apart, but separated by a difference 
equal to 1 unit of work. Since gravity accelerates a free falling mass 
about 10 meters, it performs a unit amount of work, not in 10 meters, 
or even 1 meter, but in one-tenth of a meter, and this unit is recog- 
nized as the unit distance fixing equipotential gravity surfaces, always 
measured along the plumb. A unit of work, therefore, is definitely 
fixed and unalterable, it being, in the meter-ton-second system of 
units, the amount of work equivalent to raising 1 ton vertically ? 
or about one-tenth of a meter. 
The unit work-length—viz, one-tenth of a meter (decimeter)—has 
been called by V. Bjerknes, who first used it, the dynamic decimeter; 
the other multiples being named dynamic meter, dynamic centi- 
meter, etc. It is obvious that this new measure has all the equiv- 
alents of linear measure but is restricted in its use solely to the 
vertical. The dynamic depth of any point is not the common linear 
distance of this pomt below the surface of the sea, but it is a direct 
statement regarding the amount of potential or work inherent to 
that point relative to the sea surface. 
REST. MOTION 
(a) ty 
Fic. 3.—The two states of ‘‘rest”’ and ‘‘motion”’ considered with regard to the position of the sea 
surface. (a), ‘‘rest,’’ all equiscalar surfaces, including the sea surface are level, and the entire force 
of gravity is directed as a component perpendicularly downward; (0), ‘‘motion,” the equiscalar 
surfaces, including the sea surface, are tilted, which gives rise in such surfaces to a component of 
the force of gravity and causes a movement of the water particles 
We have considered a motionless sea, and its equipotential surface. 
Suppose, on the other hand, we regard a sea surface not level; let us 
say, raised near the coast by a wind pressing the water masses up 
the inclined, continental slope. Now the sea surface being no longer 
level is, by definition, no longer of equal value potentially, and grav- 
ity exerts a component in the plane of the sea. Here we have the 
birth of a current. The size of the component force is directly pro- 
portional to the obliquity of the surface, the two conditions, “‘rest”’ 
and ‘‘motion,” being graphically illustrated in Figure 3. 
