METHODS OF RESEARCH 13 



bulb containing the mercury. A reversing 

 thermometer may be used, but a contrivance 

 must be arranged for reversing it after it has 

 taken the temperature of the water at a given 

 depth. Water-bottles must be lowered empty 

 or open at both ends, and closed at any given 

 depth from which a sample may be desired. 

 It is the same with the appliances for ascer- 

 taining the intensity and quality of sunlight 

 at a given depth : the apparatus with the 

 sensitive plates must be sent down closed, 

 then opened to expose the plates, then closed 

 again before being hauled to the surface. 



A great many of these operations are accom- 

 plished by sending messengers — small metal 

 weights — down the line to release springs or 

 to open catches. The time taken for these 

 messengers to run down a line of two or three 

 miles is very great, but with experience much 

 time may be saved by letting the messengers 

 go while the line is still running out. Another 

 contrivance is an attached propeller, which, 

 on being pulled a few fathoms towards the 

 surface, revolves and releases a spring, thus 

 closing a bottle or reversing a thermometer. 



Sounding. — During the " Challenger " Expe- 

 dition some experiments were made with 

 wire sounding lines, but fine hemp lines were 

 always used in the regular work, and in 

 very deep soundings the time when each 



