PHYSICAL OCEANOGRAPHY 



217 



sliding weight (" messenoer ") or by a propeller-release — to turn 

 upside down at the depth where the temperature is to be deter- 

 mined. The temperature is thereby regis- 

 /^^^^^\. tered, and can be read off at any time after the 



(l'i^\\ instrument has been hauled up. Aime's instru- Aime. 



ment was, however, rather intricate. In 1878 

 Negretti and Zambra of London constructed a Negretti and 

 reversing thermometer, which has played a ^'^"^b''''^- 

 prominent part in physical oceanography. In 

 this form there is a narrowing of the tube just 

 above the bulb ; the mercury fills the tube 

 above the narrowing to a greater or lesser 

 extent according to the temperature, and when 

 the thermometer is tipped over, the mercury 

 breaks off at the narrowing, the portion which 

 was above that point sinking down to the end 

 of the tube (Fig. 155); the scale on the tube 

 indicates the temperature at the moment of 

 inversion. The thermometer must be able to 

 withstand the pressure of the ocean depths, 

 and is therefore placed inside a strong glass 

 tube, with some mercury round the bulb of the 

 thermometer in order to secure a rapid conduc- 

 tion of heat. 



The Negretti and Zambra reversing thermo- 

 meter has latterly been widely used, but it has 

 been found that occasionally the mercury broke 

 off not exactly at the narrowing, but at some 

 other place in the tube, while sometimes addi- 

 tional mercury might overflow during the pro- 

 cess of hauling up. Certain improvements 

 have therefore been introduced to remedy 

 these defects, like the recent modifications by 

 C. Richter of Berlin, who altered the breaking- Richter. 

 off arrangement so as to render it quite trust- 

 worthy, and formed the tube in such a way that 

 no superfluous mercury could enter it during 

 the ascent (see Fig. 156). The severed 

 column naturally lengthens or shortens some- 

 what according to the temperature changes to 

 which it is subjected : suppose, for instance, the 

 thermometer to be reversed in water of 2.00'' C, and then 

 hauled up through warmer water and read off in the air at a 



le 



H 



4yj> 



Fig. 155. 



Negretti-Zambra 



Thermometer, 



after reversing. 



