462 WATER SUPPLY AND SEWAGE DISPOSAL 



becomes warmest, generally, not at noon, but at about two 

 o'clock. Explain this lagging of the hottest hour of the day 

 behind the time when the sun sends down its greatest heat 

 (Art. 232, page 206). 



541. Soil Temperatures. The temperature of the soil is 

 most easily taken by means of the SOIL THERMOMETERS. 



These are mercury thermometers which are usually 

 encased in wooden cases capped with metal tips (Fig. 

 282) . This protecting case makes it possible for the 

 thermometer to be pushed into the soil without 

 danger of its breaking. Repeated experiments show 

 that at the depth of 1 or 2 ft. the soil reaches its 

 highest temperature late in the afternoon or in the 

 evening. At a still greater depth, the soil reaches its 

 highest temperature during the early portions of the 

 night. The change in temperature lessens rapidly 

 as greater depth is attained until, at a depth of a few 

 feet, no appreciable daily change is noticeable. It 

 can also easily be shown that the soil at the depth of 

 1 or 2 ft. reaches its lowest temperature during the 

 day time, usually during the forenoon. This is not 

 at all mysterious; it is simply the application of the 

 FIG. 282. principle referred to in the last paragraph. If the 



Soil ther- , . - , , v i 



mometer. changes in temperature of the atmosphere lags two 

 hours behind the sun, we can easily see that the 

 changes in temperature of the soil should lag still further 

 behind. 



542. Earth's Breathing. We have seen that the soil be- 

 comes warmest during the evening and earlier portion of the 

 night. As long as it is growing warmer, the soil-air is expanding 

 and therefore rushing out. This is the EARTH'S EXHALATION. 

 The soil-air which is expelled is warm, often warmer than the 

 air above ground, and saturated with moisture. The forma- 

 tion of DEW is partly because of this warm, moist air's coming 

 into contact with the colder bodies close to the surface of the 

 ground (see Art. 253, page 224). The EARTH'S INHALATION 



