DEEP-SEx\ THERMOMETRY 25 



posed in the ocean depths, are used. We shall 

 presently come to this subject of hydrostatic pressure ; 

 for the moment, it is enough to say that at the 

 greatest depths it is sufficient to pulverise an ordi- 

 nary thermometer, and that even at moderate depths 

 it will so influence the instrument as to give us a 

 reading mainly determined, not by temperature, but 

 by pressure. 



Deep-sea thermometers are protected against this 

 pressure by having the bulb, and often the column 

 also, enclosed in another bulb or tube which is partly 

 filled with spirit. The pressure is then exerted, not 

 on the thermometer itself, but on the outer tube and 

 on the elastic spirit-vapour that fills the outer tube. 

 While, for further protection against gross mechanical 

 injuries, the whole instrument is encased in a copper 

 cage. 



When in use, the instruments are lashed on to a 

 sounding-line ; and in the Investigator no deep-sea 

 sounding was considered complete unless it was 

 accompanied by, at least, a reading of the temperature 

 at the bottom. 



Everyone knows so much about t^^awling and 

 di^edging, that it would be superfluous to speak of 

 these operations in any detail here. In the In- 

 vestigator, in my time, we almost always used, when 

 working in deep water, an x^merican reversible trawl, 



