OBSERVATIONS IN PHYSICAL OCEANOGRAPHY 



45 



been 



derived from ships* 

 B' 



average speed of the drift can be made if the bottle is picked up from the 

 water. 



Drift bottles have been used successfully for obtaining information as 

 to surface currents over relatively large ocean areas, such as the equatorial 

 part of the Atlantic Ocean and the seas around Japan. They have 

 supplied numerous details in more enclosed seas like the English Channel 

 and the North Sea, but have proved less successful off an open coast. 



The drift method can also be used for obtaining information as to 

 currents in a shorter time interval. By far the greatest number of 

 observations of surface currents have 

 records, and these data are obtained by 

 the drift method. On board ship the 

 position of a vessel, weather permitting, 

 is determined by astronomic observations 

 of the sun or stars. From this position, 

 indicated by ^ in fig. 6, the course along 

 which to continue is decided upon, but, 

 when a new ''fix" is obtained (B' in fig. 6), 

 it is in general found that the vessel is not 

 at the location B, where it should be ac- 

 cording to the course that has been steered, 

 taking the wind drift of the vessel into 

 account, and the distance covered accord- 

 ing to the log (the position by ''dead 

 reckoning"). The displacement is con- 

 sidered due to currents in the time interval 

 between "fixes." The method for com- 

 puting these currents is shown in fig. 6. 

 This method gives, in general, the average current in twenty-four hours 

 or multiples of twenty-four hours. 



From an anchored vessel, say a lightship, the surface current can be 

 determined either by a chip log or by drift buoys, below which, in general, 

 is a "current cross" acting as a sea anchor. This type of drift buoy was 

 used on the Challenger. The latter methods give nearly instantaneous 

 values of the surface currents at the place of observation. 



Near land the methods can be elaborated in such a manner that the 

 drift of a body can be determined in detail over long distances and long 

 periods. A drifting buoy can be followed by a vessel whose positions 

 can be accurately established by bearings on known landmarks, or the 

 buoy can be provided with a mast, and the direction to the buoy can be 

 observed and its distance from a fixed locality can be measured by a range 

 finder. Both methods have been used successfully. The latter can also 

 be employed in the open ocean by anchoring one buoy, setting another 

 buoy adrift, and determining the bearing of and the distance to the 



Fig. 6. Determination of sur- 

 face currents by difference be- 

 tween positions by fixes and dead 

 reckoning. 



