HYDROGRAPHICAL INVESTIGATIONS 205 



4. The gaseous contents. Sea-water always con- 

 tains a certain amount of various gases — oxygen, 

 nitrogen, and carbonic acid — dissolved in it. These 

 are dissolved from the atmosphere or derived from 

 the respiration or putrefaction of marine organisms. 

 In deep, stagnant seas the amount of oxygen at 

 the bottom may be very small. 



5. The plankton. Different regions, such as 

 the Baltic, the North Atlantic Ocean, the Arctic 

 Ocean, or the Black Sea, have characteristic 

 plankton organisms at certain periods of the year. 

 By identifying the plants and animals present in 

 any one place, it is possible to tell what was the 

 source of the water examined. 



Temperatures are determined by means of care- 

 fully constructed thermometers. If the tempera- 

 ture of the water at the sea-bottom or at any 

 intermediate depth is required, a sample is obtained 

 by means of a specially constructed bottle, which 

 is lowered down open. When it reaches the 

 depth required it is closed by means of a special 

 arrangement, and a sample of the water at that 

 depth is obtained. The bottle is so constructed 

 that while it is being raised to the surface the 

 temperature does not appreciably change. The 

 latter is then taken by an ordinary thermometer. 

 If the depth to be investigated is very great, the 

 temperature is obtained by means of a " reversing 

 thermometer," that is, an instrument which 

 registers the temperature of the water at the depth 



