METHODS OF RESEARCH 19 



introduced by the faulty formation of the 

 constriction, which may lead to the mercury 

 not always being broken off at the same 

 point. 



Now that the greatest possible accuracy is 

 called for in deep-sea work, it is not unusual 

 to send down two reversing thermometers 

 side by side in order to obtain corroboration 

 as to the actual temperature at the depth of 

 reversal. 



Water-bottles. — Recently a reversing water- 

 bottle has come into use, designed to carry 

 one or two reversing thermometers, so that 

 a temperature-reading and a water-sample 

 are secured at one and the same time (see 

 Plate I.)- Another method of attaining 

 this object is by means of insulacing water- 

 bottles, which protect the contained samples 

 from temperature-changes while being hauled 

 up to the surface, so that the temperature of a 

 sample immediately on being brought on 

 board may be supposed to indicate practically 

 the temperature at the depth where the 

 apparatus was closed. An instrument of this 

 km^ is the Pettersson-Nansen insulating 

 water-bottle. 



For obtaining a water- sample from the 

 surface an ordinary bucket may be used, and 

 in quite shallow water a stoppered bottle 

 may be sent down attached to the line in such 



