RIVERS IN THE SEA — SMITH 435 



up from below to fill the void. Similarly, when currents meet or 

 converge, there is a net displacement of water in a downward direc- 

 tion. Winds blowing away from a coast may have most important 

 effects since the water displaced offshore must be replaced from 

 below. The lower layers of water are often better supplied with 

 natural fertilizer than those at the surface so that upwellings of water 

 are apt to be more productive of sea life, including commercial fishes. 

 The west coast of Africa is a good example of such a situation. The 

 reverse case, when water is piled up along the shore, may result in 

 a sinking surface water which is replaced by less fertile surface water 

 from offshore. 



MEASURING CURRENTS 



How is it possible to measure the rate of these various currents and 

 to estimate the volume of water transported ? There is a wide variety 

 of methods available to oceanographers, some simple and some more 

 complicated, but the simplest are those that are based upon the meas- 

 urement of a drifting object. For instance, a line might be let down 

 to the bottom with a weight for anchor and the ship allowed to drift, 

 without power or sail. The rate at which the line is dragged out 

 would provide an approximate measure of the ship's movement due 

 to the current. When Columbus, halfway across the Atlantic on his 

 first voyage, tried to sound for bottom he failed to find it, but the 

 angle at which his leadline ran out from the becalmed ship gave clear 

 indication that the ship was moving westward with the surface water 

 layer while the weighted end of the line was in a deeper, relatively 

 motionless layer. Today a ship may be moored to an ice flow and the 

 current speed measured in a similar way by dropping a weighted 

 line to the bottom and measuring the speed at which it has to be 

 run out as the ship moves within the ice and current. 



CURRENTS FROM SHIP'S LOGS 



A great deal of information about currents has been obtained from 

 the navigational records of ships, filed with the U. S. Navy Hydro- 

 graphic Office. From this information the monthly averages of cur- 

 rents are charted. Using the known speed of the ship through the 

 water a navigator is able to calculate what his position should be at 

 the end of any given period of time, assuming that no currents are 

 diverting the ship. At the end of this time he finds his actual position 

 or fix by taking bearings of a stationary light or by observations of 

 heavenly bodies. The difference between the predicted position and 

 the actual position gives the direction and speed of the current. 

 Today, there are more and more reliable methods of fixing a ship's 

 position, even with an overcast sky, owing to the invention of Loran, 

 radar, and radio direction finding. Even so, this method will only 



