THE USE OF WEATHER INSTRUMENTS 197 



At Peoria, April, 1910, averaged 4 degrees warmer than April, 1911; 

 but in April, 1911, the average change from one day to the next 

 was only 3.6 degrees; while April, 1910, though a warmer month, 

 had an average changeableness of 7 degrees and included both 

 unusually warm weather and a disastrous freeze. 



All these conditions affect the crops that can be raised, the 

 industries that may be profitably carried on, and the comfort 

 and health of the population. For example: Corn requires 

 hot days, warm nights, and plenty of rain in July and early 

 August. Grass and small grains thrive better in moderately 

 cool and damp weather. People engaged in either physical 

 or mental labor can accomplish more where the temperature 

 oscillates considerably and becomes rather cool occasionally, 

 than they can where it is too warm, or even too uniform at a 

 moderate temperature. A thorough knowledge of the tem- 

 perature of a region is, therefore, important in many ways 

 (see Chaps. V and VI). 



220. Measuring the Temperature of the Air. In measur- 

 ing air temperature, three classes of instruments are used: 

 (1) Ordinary thermometers; (2) self -registering, maximum 

 and minimum, thermometers; (3) recording thermometers, or 

 thermographs. 



221. Ordinary Thermometers. Thermometers usually con- 

 tain mercury. The tubes, while open at the top, are filled and 

 the mercury heated till it completely fills the bulb and stem. 

 The tubes are then sealed. This leaves a vacuum above the 

 mercury when it cools. The scale is then placed on the stem 

 as described in Arts. 16 to 21; or the scales may be deter- 

 mined by comparing with a standard thermometer. For 

 very low temperatures, alcohol thermometers are generally 

 used. Mercury freezes at 38.5 below zero F.; therefore, 

 mercury thermometers do not record a lower temperature, 



222. Self-registering Thermometers. The Maximum. 

 In the maximum thermometer (Fig. 137) the tube is narrowed 

 near the bulb so the mercury passes with difficulty. The 

 instrument is placed nearly horizontal. When the tern- 



