A PRACTICAL METHOD FOR DETERMINING OCEAN 



CURRENTS 



Edward H. Smith 



THE ORIGIN OF CURRENTS 



In order to make a systematic exposition of the circulation taking 

 place in the oceans with especial regard to the origin of currents, 

 we have found it convenient to divide the forces into two general 

 classes: (1) Internal and (2) external. 



(1) Internal forces appear in an ocean mass whenever any change 

 takes place in the physical character of the water itself; that is, if 

 either the temperature or the salinity varies in the sea then the 

 dynamic equilibrium is upset and a tendency to readjust must follow. 

 The internal system of forces in an ocean are disturbed whenever 

 that mass radiates or absorbs heat; evaporates from the surface; 

 receives additions of fresh water; or suffers internal physical trans- 

 formation as a result of its turbulent activity. Radiation is simply a 

 gain or loss of heat by the ocean, which tends to vary the temperature 

 of the surface layers. Evaporation tends to vary the salinity of the 

 surface. The ocean receives fresh water from rain, snow, or melting 

 ice. When an ocean mixes internally it alters its physical character 

 within the region of mixing. 



(2) Forces classified as external and provocative of currents are 

 winds, tides, and variations in atmospheric pressure. The winds 

 we shall divide into two groups, determined primarily by their 

 extent and duration : (a) Those winds which by a tangential pressure 

 on the surface of the sea frictionally propagate a pure wind current 

 only; and (b) those winds which by virtue of friction drive water 

 particles against boundary surfaces in the sea and give rise to gradient 

 currents. Winds classified as (b) are by far the most important of the 

 external forces assisting to maintain the more or less prevailing system 

 of circulation in the oceans. 



There are, however, two other forces which are classified as second- 

 ary, but only in so far as they tend to deform the components estab- 

 lished by (1) and (2). They are, nevertheless, of the utmost im- 

 portance in the consideration of currents, namely, (a) the quasi 

 force due to terrestial rotation which acts simultaneously as soon as a 

 movement as described in paragraph (1) or (2) begins; and (6), fric- 



(1) 



