26o THE SEAS 



obtained keeps its original temperature, and this is then 

 read off from a thermometer fixed in the bottle itself. 

 This is a reliable method when a knowledge of the tempera- 

 ture is required only in shallow waters but a further 

 complication steps in if one wishes to study the deepest 

 water layers. 



The water at this depth is under very great pressure, 

 but when the sample has reached the surface the pressure 

 is very much reduced. Now if water is compressed its 

 temperature is slightly raised, and conversely if the pressure 

 is reduced the water becomes cooled. In the passage of a 

 sample in the insulated water bottle from a great depth 

 to the surface, the water, cold as it was at the start, will 

 be slightly colder when the observer reads the temperature. 

 This error must therefore be counteracted. Accordingly 

 a " reversing thermometer " is used, that is one in which 

 there is an S-shaped bend, so that if it be suddenly turned 

 upside down the thread of mercury is broken and a 

 permanent record of the temperature is obtained. The 

 thermometer on reaching the required depth is reversed 

 by means of a weight which slides down the wire and 

 releases a spring catch. Nowadays by using at the 

 same time both the reversing thermometer and that 

 enclosed in the insulated water bottle it has become 

 possible to compile a table from which the error due to the 

 release of pressure can be calculated, so that the water 

 bottle can always be used. At big depths a thermometer 

 enclosed in an outer case of thick glass is used as a safe- 

 guard against the pressure. 



With the insulated water bottle samples of water are 

 obtained, which, besides giving information on the tem- 

 perature, can be drawn off into clean, stoppered bottles 

 to await future chemical analysis. The water is subjected 

 to extremely delicate examinations which give information 



