OCEAN TEMPEEATUEES 353 



mation of the direction and rate of flow of water in the ocean is 

 attended with many difficulties. Some of the methods of making such 

 estimates will now be briefly reviewed. 



The most direct and widely used method is the comparison of the 

 position of a ship every noon determined from astronomical observa- 

 tion and from the log and course during the previous twenty-four 

 hours. The set of a current estimated in this way is subject to large 

 errors, unless special care is taken in making the observations. Under 

 ordinary conditions such estimates of currents less than ten miles in 

 twenty- four hours are quite uncertain (Kriimmel, 1911, p. 420). 



Another method of studying currents is to use drift bottles enclos- 

 ing slips of paper on which to enter information as to when and 

 where they were found. A sufficient number of records of the initial 

 and final positions of these bottles and the corresponding time intervals 

 will, under favorable conditions, yield information especially as to 

 the average direction of the surface drift. This method is best 

 adapted to small enclosed seas as the bottles may then be easily 

 recovered soon after reaching the shores. For the open oceans it is 

 not satisfactory. Other floating objects, such as wrecks, icebergs, trees, 

 and plankton also furnish some information about the horizontal 

 circulation. 



Under favorable conditions the current at a given place can be 

 measured directly by means of a current meter or by observing a 

 floating object designed to move with the current. In the open ocean 

 the difficulty of holding a ship in a reasonably fixed position usually 

 renders these methods impracticable. 



Investigations of the causes of ocean currents and their relation 

 to these causes provide indirect methods of determining them. Ocean 

 currents are directly due to various external forces, the wind or 

 friction of a neighboring current, differences in pressure resulting 

 from evaporation, precipitation and differences in specific gravity, and 

 are modified by the deflecting force due to the earth's rotation and 

 by internal friction of the water. Thus, any theory of ocean currents 

 capable of yielding even a rough approximation to the quantitative 

 relations between the complex system of causes and the resulting 

 motion of the water would necessarily be highly complicated. As a 

 matter of fact, great difficulties always arise in attempts to establish 

 a connection between practical hydrography and theoretical hydro- 

 dynamics, and deductions of currents from their causes are quite 

 uncertain except in special cases in which the conditions in the ideal 



