278 



DEPARTMENT OF THE NATAL SERVICE 



correspond in any way to the actual movement of the water. Dr. Hjort's section IV 

 is a good example of such abnormal distribution of density. 



By simple discussions of this kind one may soon become familiar with the various 

 phases which the state of the sea may assume, and can thus learn to determine the 

 causes by which they are produced. 



Fig. 54. — Distribution of density and cal- 

 culated velocity according to formula 

 (14) with offshore wind. 



15. THE ACCELERxiTION OF CIRCULATION. 



Let us imagine the sea and atmosphere in perfect calm and equilibrium. The 

 isosteres and the isobars coincide with each other, and with the level surfaces of gravity, 

 so that no solenoids exist. 



In a part of this motionless sea, some physical i^rocess then takes place, whereby 

 the specific gravity of the water there is altered. This gives rise to a local deformation 

 of the isosteric surfaces, which now no longer coincide with the isobars, but intersect 

 the same. The solenoids thus called into play then set the water in motion, according 

 to the formula 



dC 

 dT 



= A 



At the commencement of this movement, therefore, the acceleration of the circulation 



d C 



—- will be equal to the number of solenoids. 



d t 



As the velocity of the water increases, the resistance of friction R makes itself 



apparent. We have then 



dC 

 ~dT 



A—R 



There R increases more and more, until at last it equals A. Then, according to the 



formula, — ^ — disappears, and the movement becomes stationary. This is the sign of 

 d t 



permanent currents, originating in and maintained by a i)erpetual physical change 



taking place in the sea water, such as the Gulf Stream. 



