112 THE OCEAN RIVER 



causes and effects. Yet before we look at the north Atlantic 

 Ocean through the eyes of scientific inquiry we can better 

 understand the reasons for this impetus if we turn for a 

 moment to the growing art of navigation. In coming of age^ 

 this art took on a greater accuracy and helped to speed up the 

 ocean map-making and to usher in the infant science of 

 oceanography. 



Even today most of the charted facts about the Ocean River 

 are compiled from entries in ships' logs that show how far 

 vessels are carried from their courses by the force of water 

 currents. When the shipmaster knows the direction of his 

 course and the speed of his ship he is able to calculate the 

 apparent position at sea at any time, assuming that no currents 

 are acting. This is his ''dead reckoning." If he now finds that 

 his true position is not the same as the dead reckoning, then 

 the difference must be due to the flow of current. Thus, to be 

 able to chart ocean currents we must first be able to measure 

 the speed of a ship and then to find with accuracy its true 

 position. 



When Columbus sailed beyond known soundings the crud- 

 est kind of dead reckoning prevailed, and men set their course 

 *'by guess and by God.'' The first rude beginnings of measuring 

 speed at sea were by watching by sandglass the speed of a chip 

 of wood released at the bow of a vessel as it drifted astern. 

 After this beginning, a line was attached to a small log of wood 

 and knots along this line were counted, as it paid-out, in rela- 

 tion to a set time. Thus the word ''knot" has survived to 

 the present time as a maritime unit of speed, though this 

 kind of measure has long since been superseded by modern 

 instruments. 



There were other means of detecting and measuring the 

 pulse of the ocean currents that early explorers could well have 

 used. During his first voyage Columbus, by accident rather 

 than design, made use of a simple but effective way of meas- 



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