18 THE OCEAN 



tures, and as modified by Miller and Casella 

 was the one mostly in use during the " Chal- 

 lenger " and other expeditions of the same 

 or an earlier period. This thermometer is 

 satisfactory for work in the open ocean, where 

 the temperature as a rule decreases gradually 

 from surface to bottom. Towards the polar 

 regions and in some enclosed seas, however, 

 layers of different temperatures may lie one 

 above another, and such conditions would 

 not be disclosed by the Six form of thermo- 

 meter ; it has, therefore, been replaced to a 

 large extent by another form, which may be 

 reversed, and the temperature thereby regis- 

 tered, at any desired depth. 



The reversing thermometer (see Plate I.) 

 is sent down with the bulb lowermost ; there 

 is a narrowing of the tube just above the bulb, 

 and the length of the column of mercury above 

 the constriction depends upon the temperature. 

 On being reversed the mercury is broken off 

 at the constriction, the bulb being now upper- 

 most, and the colunm of mercury that was 

 above the constriction falls down, the tempera- 

 ture at the moment of reversal being read off 

 in the reversed position. The reversing 

 thermometer has been modified and improved 

 by Negretti and Zambra and by Richter, so 

 that at the present day it is a very efficient 

 instrument. Occasionally an error may be 



