METHODS OF OCEANOGRAPHICAL RESEARCH 257 



Of equal importance in navigation comes the necessity 

 of knowing the speed and direction of the ocean currents. 

 The bearing of currents on the distribution of the life 

 drifting m the water layers is a problem that has received 

 considerable attention in modem fishery research. Informa- 

 tion can be gained about currents both directly by noting 

 the movements of floating and drifting objects or actually 

 measuring the speed and direction by special instruments, 

 and indirectly by a study of the physical and chemical 

 condition of the water itself from place to place. 



In olden days much of the knowledge gained about ocean 

 currents was acquired by the mariners themselves noting 

 the position of drifting wreckage or derelict ships and as 

 reports came in from different vessels it was possible to 

 mark off on a chart the route taken by a drifting wreck 

 and therefore to gain some idea of the general trend of 

 the current. There was however a danger in basing too 

 definite a conclusion on such observations, as the wreckage 

 always had a portion above the water level exposed to the 

 winds so that the path taken by such a wreck was therefore 

 caused by a combination of wind and current. To-day 

 information is sought by using specially constructed 

 bottles which are thrown overboard at specified positions. 

 They are known as " drift bottles " and they are used to 

 study the water movements at the surface or near the sea 

 bottom. The surface bottles are so weighted that they 

 float almost completely submerged, and thus the least 

 possible area is exposed to wind action. The bottom 

 bottles are made just the slightest bit heavier than water 

 and have fixed to them about eighteen inches of stiff wire ; 

 this wire trails along the bottom and holds the bottle 

 clear of stones and other obstructions that might impede 

 its movement as it is carried by the current (Plate 90). 

 In each bottle is placed an addressed postcard with 



