RECORDING TEMPERATURES 



In addition to the maximum and minimum thermo- 

 meters, there were the wet and dry bulb thermometers. 

 The dry bulb records the actual temperature of the air 

 at the moment, and we used a spirit thermometer for 

 this purpose. The wet bulb consisted of an ordinary 

 thermometer, round the bulb of which was tied a 

 little piece of muslin that had been dipped in water 

 and of course froze at once on exposure to the air. 

 The effect of the evaporation from the ice which covered 

 the bulb was to cause the temperature recorded to be 

 lower than that recorded by the dry bulb thermo- 

 meter in proportion to the amount of water present 

 in the atmosphere at the time. To ensure accuracy 

 the wet bulb thermometers were changed every two 

 hours, the thermometer which was read being brought 

 back to the hut and returned to the screen later freshly 

 sheathed in ice. It was, of course, impossible to wet 

 the exposed thermometer with a brush dipped in water, 

 as is the practice in temperate climates, for water could 

 not be carried from the hut to the screen without 

 freezing into solid ice. To check the thermometers 

 there was also kept in the screen a self-recording ther- 

 mometer, or thermograph. This is a delicate instrument 

 fitted with metal discs, which expand or contract 

 readily with every fluctuation of the temperature. 

 Attached to these discs is a delicately poised lever 

 carrying a pen charged with ink, and the point of this 

 pen rests against a graduated roll of paper fastened 

 to a drum, which is revolved by clockwork once in every 

 seven days. The pen thus draws a line on the paper, 

 rising and falling in sympathy with the changes in the 

 temperature of the air. 



All these instruments were contained inside the 

 meteorological screen, which was so constructed that while 

 there was free access of air, the wind could not strike 



201 



