SURVEYING THE ROUTE. 23 



the bulb. Referring to Fig- 15 it will be seen that the 

 mercury column has a peculiar bend a little above the bulb. 

 It is curved and narrowed down at A, widened into a small 

 chamber at B, and curved again. At ordinary temperatures 

 the mercury column is somewhat above this chamber, the 

 thermometer standing vertically with the bulb downwards, as 

 in the figure. In this position at high temperatures the 

 mercury may rise and partially fill the reservoir C. When it is 

 desired to register the temperature at any particular place the 

 thermometer is turned upside down. The mercury in the 

 column above the contracted portion A then falls to the other 

 end of the stem, and so long as the instrument is kept in an 

 inverted position no more mercury can fall through, thus 

 keeping a correct record of the temperature at that particular 

 spot. Should the instrument be subsequently subjected to a 

 higher temperature than that registered, the mercury can 

 expand into the chamber B and lodge there without afiecting 

 the indication. The higher temperature would, however, aflfect 

 the separate portion somewhat, but not to any great extent 

 on account of the small mass of mercury. The column is a 

 thin flat one, not circular in section. For strict accuracy, 

 however, there should be a correction table for applying to the 

 readings to eliminate the eflfect of the temperature at which the 

 instrument is read. The scale of degrees is inverted as the 

 temperature recorded is read with the instrument upside 

 down. 



The thermometer is enclosed in a glass protecting tube as 

 shown in Fig. 15. There is an air-tight cemented rubber 

 partition fixed at D below which mercury is filled in to a 

 certain height, the remaining space being exhausted of air. 

 This is done by filling the whole of the space with hot mercury, 

 which when cooled down leaves a vacuum. The mercury 

 jacket round the thermometer bulb forms a heat-conducting 

 medium which renders the protected bulb very sensitive to 

 external changes of temperature. Under deep-sea pressures 

 the outside tube is compressed, and the space within the 

 jacket correspondingly diminished, but as the jacket is 

 partially exhausted of air this compression is not transmitted 

 to the thermometer bulb, and therefore the instrument itself is 

 protected, and its indications freed from any error due to 



