Chapter XII 



The Representation of Oceanic 

 Movements and Kinematics 



1. Methods of Observation and Measurement of Oceanographic Currents 



Two different methods can be used to determine the nature of the currents in the sea. 

 One follows the Lagrange approach and investigates the track which a small element 

 of water follows in time. This gives the trajectory of the water movement from the 

 sequence of points in space through which the water element passes. The other 

 method using an approach closer to that of Euler considers the current from a fixed 

 point, and shows the nature of the current at a fixed point at any particular moment 

 in terms of the current vector, which is variable with time. Graphic representation of 

 the distribution of velocity in space by fines of equal intensity (isotachs, velocity 

 field), or by representing the directional field by means o^ stream lines (see p. 326). The 

 stream lines and the velocity field fix the current field at any particular instant. 



The trajectories and stream lines must be carefully distinguished; they will coincide 

 only in the case of a steady current and here the stream line will also be the same as 

 the trajectory taken by a small water element. 



(a) Drift Bottles and Drifting Objects 



A more or less accurate indication of the direction and velocity of water currents 

 can be obtained by following the drift of objects of all sorts which may temporarily 

 or permanently be floating in the water, whether through change or through having 

 been placed there deliberately by man (Krummel, 1908). It is essential that these 

 drifting bodies should project as little as possible out of the water so as to minimize 

 the important influence of wind and waves on their displacements. 



The course followed by drifting bodies of this sort, which are subject only to the 

 effect of the currents, gives the trajectories of the water movement. Floating bodies 

 put into the sea especially for this purpose may also be used {drift bottle, bottle post). 

 On account of their cheapness and simple handling drift bottles have been frequently 

 used, and with systematic and methodical work can give useful results. Since the path 

 followed by a drift bottle depends to a considerable extent on chance, unambiguous 

 results are given only by systematic work and by the use and recovery of a large 

 number of such bottles. Large-scale experiments of this type have been made by 

 Prince Albert I of Monaco (1889) in the eastern North Atlantic, by Fulton (1897) 

 in the North Sea and more recently, with particular success, by Carruthers (1954) 

 in the southern part of the North Sea and the English Channel. 



The ordinary drift bottles usually give only the starting position and the place of 



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