48 A TEXTBOOK OF OCEANOGRAPHY 



mercury back into the left-hand branch, where there is another 

 index-pin. 



In this way the maximum and minimum temperatures are 

 registered. Quite good records were obtained with this 

 thermometer. ^ 



The best modern deep-sea thermometer is of the reversing 

 type. Of these there are several varieties, those of Negretti- 

 Zambra, Knudsen, Chabaud, and Richter being the chief. 

 All are similar in principle. 



In the reversing thermometer there is a narrowing of the 

 tube just above the bulb. The mercury rises and falls in the 

 tube above the narrow portion according to the temperature. 

 When the thermometer has been lowered to the required depth 

 it is reversed by means of a messenger sent down the wire. 

 This causes the mercury to break off at the narrow point, the 

 column of mercury above this point sinking down the tube, 

 where it is now isolated from the mercury in the bulb. To 

 enable the thermometer to withstand the strong pressure at 

 great oceanic depths it is surrounded with a strong glass tube 

 with mercury enclosed around the bulb to act as a rapid 

 conductor of heat. 



The severed column lengthens or shortens according to the 

 change in temperature which occurs in hauling the instrument 

 inboard. This change is calculated if the temperature at the 

 time of reading off is known, and consequently it is now 

 customary to enclose a small thermometer inside the strong 

 glass tube, the reading of which gives the data for the 

 correction. 



It is now possible with the best types of reversing ther- 

 mometers to determine the temperature of sea-water to T ^ 

 degree Centigrade. 



