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 (b) 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 tlu-ee variables 

 separately and their combinations as they lead to dynamic measiure- 

 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 M, L, 

 and T, respectively, and m 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. Z = length, L^^^rea, Z,^ = volume. If we consider any 

 mass with respect to unit volume we then are determining density, 



or q = Y3^ ML~^. But inversely, if we contemplate a volume with 



respect to unit mass, the result is termed specific volume, or 



v = -jy=DM-\ If we divide a length by a time then it gives rise 



to a consideration of motion called velocity, or c = yp=LT~^; con- 

 tinuing to divide a velocity by a time (rate of rate of motion) is called 



c L 

 acceleration or a = yp= -Fp2 = L T~^. 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 substitutmg a = LT-\ we get 1= MLT-\ If M is 



