30 ELEMENTS OF AGRICULTURE 



start up other slow-moving heat" waves in the air press- 

 ing against the earth's surface. The heat waves started 

 in the air tend to rise, but the moist, dense air near the 

 earth's surface checks their upward progress and holds 

 them till the air becomes thoroughly warmed by their 

 motion. Rising slowly th rough the dense air near the 

 earth, the "heat waves finally reach the thin upper air 

 through which they pass very easily, and are soon lost 

 in outer space. High mountain tops are surrounded 

 by thin air, through which the heat waves from the 

 earth pass so rapidly that it never becomes thoroughly 

 warm, consequently they are always cold. The dense 

 layer of air near the earth's surface acts like a heavy 

 blanket or- covering; the thin air of mountain tops is a 

 very poor covering. 



Dew and frost seldom occur on cloudy nights, because 

 the moisture in the clouds holds the heat waves from 

 the earth, which keep the air warm and prevent the 

 condensation of water vapor. Dust and smoke have 

 much the same effect as clouds, and for this reason 

 smoke is sometimes used to prevent frost. Wind also 

 prevents frost, the air being kept in such rapid motion 

 that little or no moisture is deposited. Moisture, dust, 

 and smoke all tend to prevent radiation. In dry air 

 radiation takes place rapidly, and consequently desert 

 countries are nearly always cold at night, though warm 

 in the day. ■ 



31. Solid Substances in Air. — Dust particles may 

 often be seen in the air, and especially is this true in 

 dry weather, when they become so numerous as to be 

 very disagreeable. Rain washes most of the dust from 



