138 THE OCEAN RIVER 



almost exactly identical with those definitely measured by 

 Pillsbury during his long periods at anchor in the Florida 

 Straits between 1885 and 1889. 



The slow but steady growth of information and under- 

 standing which sprang from the birth of oceanography along 

 the Ocean River's shores has encompassed not only the surface 

 streams but also another movement, more massive though 

 less obvious, deep down below the surface of the open ocean. 

 We have mentioned in a previous chapter that in every second 

 of time the south equatorial drift carries six million tons of 

 water across the equator into the North Atlantic. There is 

 no appreciable return of surface water to the South Atlantic; 

 hence there would be a continuous piling up of water north 

 of the equator and a continuous loss of water from the south- 

 ern ocean unless some other path existed for a compensating 

 southward movement. It is now known that deep below the 

 surface such a movement of water does take place. Deep 

 unseen flows of heavy, ice-cold water are fortunately fairly 

 easy to trace, because it has been found that large masses of 

 water differ from each other in their relative degrees of tem- 

 perature and saltness. Thus, even when it is difficult to 

 measure the rate of flow, oceanographers can still identify the 

 sources of the deep waters by sending down their sample 

 bottles on thin wire cables. 



The sample bottles are made of steel tubing, open at both 

 ends, so that on their way down water may pass freely through 

 them. When they reach the required depth a messenger 

 weight, slid down the cable, trips a release catch on the bottle, 

 which then traps a sample of the water. At the same time an 

 extremely accurate thermometer, attached to the bottle, is 

 reversed in such a way that the thread of mercury is inter- 

 rupted and the temperature of the deep water may still be 

 read when the bottle returns to the surface. In order to save 

 time, ten or more bottles with their thermometers may be 



