CHAPTER XIII 

 TEMPERATURE OF THE AIR 



The temperature of the air varies from season to season, from 

 day to day, and even from one part of a day to another. It is so 

 important in human affairs, that it is convenient to have some 

 easy way of measuring and recording it. 



The thermometer. The temperature is measured by the ther- 

 mometer, which consists of a glass tube of uniform diameter except 

 for a bulb at one end. The bulb and the lower part of the tube are 

 filled with some liquid, generally mercury, and this is heated until 

 it boils. The boiling expels all air, and while the mercury is boil- 

 ing the tube is sealed, the heat being withdrawn at the same 

 moment. On cooling, the mercury contracts, and fills the lower 

 part of the tube only. Whenever the temperature rises, the mer- 

 cury expands and rises in the tube, and when the temperature falls, 

 the mercury contracts and sinks. The amount of rise or fall of the 

 mercury shows the amount of change of temperature. 



A scale is marked on the tube so that the temperature may 

 be read from it. Two scales are in common use the Fahrenheit 

 and the Centigrade. The scales are marked on the tube as follows: 

 The thermometer tube is placed in boiling water, or in steam just 

 over boiling water, at sea-level (760 mm. or 14% Ibs. pressure, 

 see p. 262), and allowed to stay there until the tube and its con- 

 tents have the temperature of the water. The point to which the 

 mercury rises in the tube under these conditions is marked 212, 

 if the Fahrenheit scale is to be used. The tube of mercury is then 

 put into pounded ice or snow at a melting temperature, where it 

 remains until the level of the mercury in the tube stops sinking. 

 The level at which the mercury then stands is marked 32. The 

 space between the 212 mark and the 32 mark is divided into 180 

 equal parts, each being called a degree (1 Fahr.). The marks on 



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