NATURE OF THE STREAM 135 



between them, and in this way motion from one layer is 

 quickly transferred to the next layer and so on. This inde- 

 pendent study brought new support to the wind-driven theory 

 of ocean circulation, by showing that the ocean when driven 

 by winds at the surface does not require thousands of years to 

 build up a deep current, but a very much shorter time. The 

 eddies acting on each other engage like invisible gears in mesh 

 — a kind of fluid drive. 



Meanwhile another concept of the physicists had for over a 

 century grown independently of oceanography before scien- 

 tists saw its application to the mysteries of ocean currents: 

 this was the effect of the earth's spin. From the very beginning 

 curiosity about the causes of the Ocean River had led men to 

 a dim realization that the earth's rotation played some part in 

 their movement; but the early ideas of Kepler and others 

 entirely neglected one of the most important effects — the 

 Coriolis force. This was explained mathematically by Laplace 

 as early as 1775, but it was not applied to the ocean until 

 toward the end of the nineteenth century. At first the grow- 

 ing interest in wind and weather inspired men to study the 

 effects of the Coriolis force on the wind systems, and its true 

 relation to water currents was made clear only as late as 1903 

 by the pioneer oceanographers Sandstrom and Helland- 

 Hansen. 



What is this mysterious Coriolis force that pulls wind and 

 water into a clockwise spin in the northern hemisphere and 

 contrariwise south of the equator? The answer is that in the 

 ordinary sense there is no force. There is simply a tendency 

 for moving objects to continue in the same straight line, com- 

 bined with the fact that straight-line movements on the earth's 

 surface go in circles. To understand this paradox it is only 

 necessary to spin a cardboard disc slowly on a phonograph 

 turntable, and then, paying no attention to the fact that the 

 disc is turning, draw a pencil across it in a straight line from 



