356 



The Representation of Oceanic Movements and Kinematics 



From, May 27 1895- June 26. 1896 



Fig. 148. Dependence of the ice drift on the wind direction according to the observations 

 of the "Fram" expedition, 27 May 1895 to 27 June 1896. Left side: the observed total ice drift. 

 Right side: the pure wind drift after subtraction of the effect of the permanent basic current 



(according to Sverdrup). 



the wind direction. Also in this case almost identical values were obtained by Nansen's 

 method. 



Palmen (1930/)) has studied these methods in his work on the currents of the Gulf 

 of Bothnia and the Gulf of Finland more deeply and has used them with success, 

 especially for the observations on wind and currents made at the light ship "Finn- 

 grundet" from 1923 to 1927. 



2. The Current Field and its Representation 



{a) Representation of Mean Current Conditions by Means of Compass Cards 



To get an idea of the currents in any particular area of the sea the most practical 

 procedure is to tabulate all the available data for the direction and strength of the 

 currents for small areas over which uniform conditions can be expected. These small 

 areas are usually chosen to cover a few degree squares (one, two or more degree 

 squares). The question is thus to count out a large number of observations which 

 can then be presented on a compass card. The prevalence of each direction is then 

 shown by longer or shorter rays from the centre point, and the mean velocity in any 

 direction is shown either by the thickness of this line or by the feathering on these 

 rays. Such a current chart is actually only a graphical tabulation and is very largely 

 free of subjective influences. A personal factor becomes involved only in the interpre- 

 tation of the picture shown by such compass cards. 



The representation of current conditions by compass cards best satisfies the require- 

 ments of a current chart for navigation, since it gives at a single glance the frequency 

 and strength of currents in each direction and the possibility of representing large 

 variations in the direction and strength of the current. The usefulness of charts con- 

 taining compass cards for scientific investigation of the sea is, however, very limited, 

 because sufliicient observations are available only along shipping routes and there are 

 larger areas of the sea for which cards cannot be constructed due to missing data. 

 The use of compass cards to show average current conditions was previously pre- 

 ferred, and by this a uniform evaluation of the enormous amount of ships reckoning 

 displacements was made. One of the most recent representations using compass 

 cards is that of the Netherlands Atlas for East Asian waters {Kgl. Ned. Met. Inst. 



