6 
An example has been given when, by the absorption of heat, the 
water becomes lighter over a coastal shelf in summer. 
(2) External class of forces can not possibly produce the slightest 
physical change in the character of the water particles themselves 
(when the turbulent effect of the wind is disregarded), but either 
they directly drive the water particles in a current or they deform a 
water mass that is qualified by boundary conditions. The latter 
type. similar to (1), tends to vary the distribution of density in the 
sea; an example has been given in the case of an onshore wind piling 
up the lighter surface water against a coast. 
Thus we may sum up the distinction between the two classified 
origins of currents—viz, class (1) forces tend to alter the physical 
character of the sea water while class (2) forces tend either (a) to 
move the water particles in a current or (6) to deform eventually a 
given water mass. 
THREE VARIABLES IN THE SEA 
It is best to begin by treating the distribution of density in the 
light of mechanics and physics. We may regard each type as being 
a field of strain inherent to the mass itself, an effect of stresses, the 
fields of which in the sea can be treated when expressed in terms of 
three variables classified as follows: (1) Gravity, (2) pressure, (3) 
specific volume. Let us examine each one of the three variables 
separately and their combinations as they lead to dynamic measure- 
ment of currents. 
First, however, it will be helpful to review some of the fundamen- 
tals elementary to a physical science. The three fundamentals in 
physics are MASS, LENGTH, and TIME, represented by the letters YU, L, 
and 7’, respectively, and in these terms we may express any form of 
physical phenomena belonging to the sea. If a length, which is the 
most tangible of the three, be squared, the result is an area; if cubed, 
a volume. L=length, [*=area, L?=volume. If we consider any 
mass with respect to unit volume we then are determining density, 
or a= = ML-*. But inversely, if we contemplate a volume with 
respect to unit mass, the result is termed specific volume, or 
3 
ERAT! . 
to a consideration of motion called velocity, or C= m= LT, con- 
=I? M-'. If we divide a length by a time then it gives rise 
tinuing to divide a velocity by a time (rate of rate of motion) is called 
acceleration or a= 7 = =LT-. A force is that agent which gives 
motion to a mass. It is expressed in a measurement which considers 
the mass relative to its rate of change of motion—i. e., acceleration. 
K= Ma; but substituting a=LT~*, we get k= MLT~. If Mis 
