CHAPTER V 



OCEAN CURRENTS 



THE commonest method of detecting the existence of ocean 

 currents is by ship's reckoning. The master of every ship is 

 supposed at midday to determine his position by means of 

 astronomical calculations based on the observed height of the 

 sun. He is also in a position to calculate his course and the 

 distance travelled since the preceding observation of yester- 

 day. The difference between the positions determined by 

 astronomical observation and by course and log gives an 



17 



13 



//" IB 75 /*' 



FIG. 23. SURFACE CURRENTS : ESTIMATION BY SHIP'S RECKONING. 



indication of the ocean currents in the locality traversed. As 

 an example, suppose a vessel at noon of a certain date is at the 

 point A in the diagram (Fig. 23). A course is steered in a 

 south-easterly direction, and as a result of twenty-four hours' 

 steaming 180 miles are logged, so that the position of the 

 steamer is B'. 



An astronomical observation shows that the true position 

 of the ship is, however, B. 



