84: SCIENCE OF COMMON THINGS. 



Why clouds float in the air. Height of clouds. Size of clouds. 



2. The air itself (on a fine day) retains much of its 

 vapor in an invisible form. 



537* Why do clouds float so readily in the air f 



Because they are composed of very minute globules 

 (called vesicles), which (being lighter than air) float 

 like soap-bubbles. 



538 Are att clouds alike f 



No ; they vary greatly in density, height, and color. 



530 What is the chief cause of fog and clouds f 



During the daily process of evaporation from the 

 surface of the earth, warm, humid currents of air are 

 continually ascending ; the higher they ascend, the 

 colder is the atmosphere into which they enter ; and, as 

 they continue to rise, a point will at length be attained 

 where, in union with the colder air, their original 

 humidity can no longer be retained : a cloud will then 

 appear, which increases in bulk with the upward pro- 

 gress of the current into colder regions. 



540 How do changes in the wind produce clouds f 



If a cold current of wind blows suddenly over any 

 region, it condenses the invisible vapor of the air into 

 cloud or rain j but if a warm current of wind blows 

 over any region, it disperses the clouds by absorbing 

 their vapor. 



541 What distance are the clouds from the earth f 



Some thin, light clouds are elevated above the high- 

 est mountain-top ; some heavy ones touch the steeples, 

 trees, and even the earth ; but the average height ia 

 between one and two miles. 



Streaky, curling clouds, like hair, are often five or six miles high. 



542 What is the size of the clouds f 



Some clouds are many square miles in surface, and 

 above a mile in thickness / while others are only a few 

 yards or inches. 



543 How can persons ascertain the thickness of a cloud f 



As the tops of high mountains are generally above 

 the clouds, travellers may pass quite through them into 



