no OCEAN CURRENTS RELATED TO THE DISTRIBUTION OF MASS 



are found if the reference level is placed at a sufficiently great depth. 

 This method is useful if a study of the water masses substantiates the 

 view that the motion of the deep water is negligible. 



A second school of oceanographers claims that the distribution of 

 oxygen in the ocean must be closely related to the type of motion, and 

 especially that the layer or layers of minimum oxygen content that are 

 found over large areas must represent layers of minimum horizontal 

 motion, but this concept is finding a more and more limited application. 



A third method has been employed by Defant, who points out that in 

 the Atlantic Ocean the relative distances between isobaric surfaces 

 remain nearly constant within certain intervals of depth. He assumes 

 that a surface of no motion lies within this interval, and arrives in this 

 manner at a consistent picture of the shape of the reference surface in the 

 Atlantic and at results which are in good agreement with those obtained 

 by considering the equation of continuity. 



A fourth method is based on the equation of continuity, but this 

 method has so far been little used because it requires comprehensive data. 

 The application can be illustrated by considering the currents of such an 

 ocean as the South Atlantic. It is evident that the net transport of 

 water (p. 115) through any cross section of the South Atlantic betw^een 

 South Africa and South America must be zero, because water cannot 

 permanently be removed from the North Atlantic or be accumulated in 

 that ocean, which is practically a closed bay. In the South Atlantic a 

 surface of no motion must therefore be selected in which the flow to the 

 north above that surface equals the flow to the south below the same 

 surface (p. 116). Similarly the surface of no motion has to be selected 

 in other regions in such a manner that one arrives at a consistent picture 

 of the currents, taking into account the continuity of the system and 

 the fact that subsurface water masses retain their character over long 

 distances. Examples of such pictures are shown in figs. 42 and 56. 



In many regions the surface of no motion does not coincide with a 

 level surface, and in these cases the topographies of isobaric surfaces have 

 to be constructed stepwise by considering that the isobaric surfaces must 

 be horizontal where they cross the surface of no motion. 



From the many reservations made, it may appear that the compu- 

 tations reach only rough approximations to the actual conditions. How- 

 ever, in many instances in which comparisons are possible it has been 

 found that the computed currents do not deviate much from the observed 

 ones. As an example, in fig. 26 a comparison is shown between the 

 velocities of the current through the Straits of Florida according to 

 Wiist's computations and according to Pillsbury's detailed observations 

 in the 1880's. The observed distribution of temperature and salinity 

 and the corresponding currents are shown in parts A, B, and C, and in 

 part D are shown the observed currents. The agreement is striking. 



