The Representation of Oceanic Movements and Kinematics 



349 



hour. These findings by Thorade indicate that the effect of the movements of the 

 vessel from which the measurements are made and other chance factors can be eUmin- 

 ated by such a smoothing procedure. Instead of using continuous recordings of the 

 current followed by calculation of the mean over a long interval such equipment is 

 used in practice which gives directly mean values for the direction and velocity over 



R -20 



■30L 



,830 1835 1840 1845 1850 |855 19OO 

 IO-2ni-l93l 



-10 



,E, -20 



IO-5fflI-l93l 



Fig. 143. Upper picture: mean for each minute; lower picture: mean for each 5 min of the 



north ( \ \ ) and the east component (— O — O— ) of the current measured from 



the "Poseidon" (see Fig. 142). 



a longer interval (10 min or more). In deriving the means it should be remembered 

 that they are vectors and in order to reduce them to mean values they must be re- 

 solved into north and east components. The mean obtained in this way is denoted 

 the vectorial mean. Instead of this mean, which is mathematically accurate but in- 

 convenient to calculate, the mean of all the velocities regardless of the direction is 

 often used instead. This is termed a scalar mean of the velocity, and it represents the 

 average velocity of the water displacement. The corresponding simple arithmetic 

 mean of the angle of the flow direction is of no importance especially when the 

 variations in the direction are large. 



In the characterization of extensive current measurements a further quantity is 

 used to give a numerical value for the variations in direction and speed of the current. 

 The quotient of the vectorial velocity mean and the scalar mean is used for this and is 

 termed the constancy (stability) of the current {Kgl Ned. Med. Inst. De Bilt, 1904, 

 1908). From the definition of the two kinds of averages it follows that the stabiHty 

 is always a proper fraction. It has the value 1 if the directions of the individual vectors 

 are always the same size, since the vectorial mean is then the same as the scalar. The 

 current constancy is usually expressed as a percentage. 



