ECHO-SOUNDING AND H YDROGRAPHICAL STUDIES 



37 



bottle is another weight, and the reversing of the bottle releases this, which 

 slides down the wire and releases in turn another bottle. The process may be 

 continued, so as to give temperatures and water samples from anything 

 between five and lo depths in one operation. 



Reliable reversing thermometers and water bottles of this kind have 

 been the principal means by which expeditions such as the Meteor, the 

 Dana, and the Snellius have provided us with knowledge of the hydro- 

 graphy of the oceans even at great depths. 



The sea, like the atmosphere, is divided into two strata. The rather 

 narrow upper stratum is called the thermosphere, the rest down to the 

 bottom being the psychrosphere. In the thermosphere are located all the 

 currents which we observe at the surface and which are mostly driven 

 by the prevaihng winds; for instance, the great circumpolar current 

 flowing west to east southward of the continents, the equatorial currents 

 flowing east to west on both sides of the Equator, the GuK Stream in the 

 Atlantic, and its counterpart the Kuro-Siwo in the north Pacific. As the 

 thermosphere currents are only 400 metres deep at most, the Galathea 

 Expedition was chiefly concerned with the hydrography of the psycro- 

 sphere. 



It is a common feature of all the oceans 

 that psychrospheric water moves very slowly, 

 so that water masses bearing salts and oxygen 

 which have sunk down from the thermosphere 

 may take years to move from one ocean re- 

 gion to another. 



The three world oceans all extend south- 

 ward to the Antarctic coast. In a way they 

 may be regarded as three vast bays of a great 

 southern, circular ocean, up to 4,000 — 5,000 

 metres deep, which forms a mutual connec- 

 tion. Owing to their different sizes and their 

 connections with various secondary seas they 

 have their own special hydrographical cha- 

 racters; but because the currents of this sou- 

 thern circular ocean, from surface to bottom, 

 hourly carry about 350 cubic kilometres of 

 water in an easterly direction, the three oceans 

 strongly influence one another. 



The northward extension of the Atlan- 

 tic (to Greenland, Iceland, and the Faroes) 



Reversing bottle coming up in 

 released position. 



