344 



The Representation of Oceanic Movements and Kinematics 



47° 



46° 



45° 



44° 



43° 



42° 



Fig. 139. Drift method for the determination of surface currents by the difference between 

 the astronomical position and the position according to dead reckoning. Ship's displacement : 

 A, position at noon of the previous day; A^, astronomical position; A^, position according 

 to dead reckoning; A^A^, ship's displacement; BA^, difference in latitude; BA2, difference 



in longitude. 



any point will therefore give the true mean current at that point. However, this of 

 course will only apply when the current is more or less a steady one. 



(c) Current Measurements 



Ship displacements give only the mean values of the currents over 24 h. If the in- 

 stantaneous value of the current or a continuous record at one position is required 

 then current measurements will be necessary. These will give the direction and strength 

 of the current, both at the surface and also in layers beneath it. For measurements of 

 this type at any point 3. fixed reference-position is needed. It is thus necessary to anchor 

 the vessel from which the observations are to be made. In shallow waters this oifers 

 no difficulty but at great depths the difficulties increase considerably and a special 

 technique and anchoring equipment are required. 



If the vessel is firmly anchored and the anchor holds it is not necessarily a fixed 

 reference-point from which current measurements can be made directly without more 

 ado. Any ship anchored with a long cable will be subject to movements due to the 

 changes in the wind and the current, and these movements can have considerable 

 effect on the current measurements made from the vessel. Three types of ships' 

 movements can be distinguished (Defant, 1932, p. 7). The first two types, swinging 

 round (Schwoien) and swinging (Schwingen) are shown by changes in the heading of 

 the ship with time and can be determined by continual readings of the ship's compass. 



