132 A TEXTBOOK OF OCEANOGRAPHY 



It follows that the steamer on its voyage from A in a 

 direction towards B f has met with some force which has 

 pushed it out of its direction and shortened its course. This 

 force is the ocean current of the locality, and the displacement 

 of the steamer is represented by the line B'B. This can be 

 measured, both in magnitude and direction. Let us suppose 

 that it is 26 sea miles and in direction west by north. This 

 gives us the velocity and direction of the current for twenty- 

 four hours. Near the Equator the estimation is simple, but in 

 higher latitudes other calculations are necessary. 



In observations of this kind there are many sources of 

 error, so that one isolated record is not of much value. 



CURRENT METERS. 



(A) Surface. 



(B) Deep-sea. 



Surface Meters. 



Modern determination of ocean currents depends on the 

 use of various instruments; the principal types only are 

 described here. 



In the Challenger expedition floating buoys were used, 

 and to these were attached by means of a line, two frames of 

 sail-cloth arranged at right angles and kept immersed by 

 means of a lead weight. When the apparatus was not in use 

 the frames could be folded together. The actual determination 

 of surface currents by a ship anchored at sea is attended with 

 considerable difficulty.* If one anchor alone be used, the ship 

 sways to and fro to a great extent, thus hindering the observa- 

 tions. Three anchors are found to be necessary to give a 

 satisfactory result, and it will be easily understood that to 

 anchor a ship in the open sea for a sufficiently long period 

 to enable current observations to be made is an expensive 

 process, and one not unattended with danger. 



* See Pillsbury, U.S. Coast Survey Report for 1890, Appendix X., 

 pp. 516-537. Washington, 1891. 



