CH. l] THE EXPLORATION OF THE SEA 11 



thermometer was one in which a long time was required for the 

 instrument to indicate the temperature of the medium in which it 

 was placed. Therefore in hauling it to the surface the temperature 

 did not greatly change and the time increments of change could 

 be calculated. But a practical objection was the time required to 

 make an observation. So this method also has been abandoned. 

 Finally we have the reversing thermometers, and these are still 

 used. The instrument of this class originally made by Negretti 

 and Zambra, Fig. 3, is too well known to require description. 

 We have a thermometer tube which is constructed in much the 

 same manner in which a clinical thermometer tube is made, that 

 is the bore of the tube immediately above the bulb is bent twice 

 in an S-shaped curve and is greatly contracted at this point. In 

 a clinical thermometer the thread of mercury breaks when the 

 instrument is taken out of the place the temperature of which is 

 desired, and when the metal contracts with the reduction of 

 temperature. The mercury in the capillary tube therefore remains 

 where it was. In a deep-sea thermometer in which this principle 

 is adopted the case carrying the tube is reversed when the 

 desired depth is attained, this being done usually by a small 

 propeller in the outer frame of the instrument which revolves 

 immediately the direction of movement of the machine is changed 

 on being hauled, and then releases a catch which allows the tube 

 to reverse. The mercury then falls to the other end of the tube 

 and as that latter is graduated from this end the temperature is 

 recorded. In the Nansen-Pettersson water bottle a reversing 

 thermometer is usually attached to the frame of the instrument 

 and is reversed when the messenger springs a catch at the same 

 time as closing the bottle. But since the thermometer enclosed 

 in the central chamber of the bottle takes the temperature of the 

 water in situ, and since this sample of water does not change 

 greatly in temperature because it is surrounded by a practically 

 non-conducting material, the indications of the reversing instru- 

 ment are only used as a check on the other. The sample of water 

 obtained by the Nansen-Pettersson bottle does actually change in 

 temperature as the instrument is being hauled through water of a 

 temperature markedly different from that to which it has been 

 lowered, but the change is very slight and can be allow^ed for. 



