SCIENCE OF COMMON THINGS. 85 



Cause of the appearance of clouds. Color of clouds. 



a clear blue firmament, when the clouds will be seen 

 beneath their feet. 



544 Why do clouds, when not continuous over the whole surface of the 

 sky, appear jagged, rough, and uneven ? 



The rays of the sun, falling upon different surfaces at 

 different angles, melt away one set of elevations, and 

 create another set of depressions ; the heat also which is 

 liberated from below in the process of condensation, 

 the currents of warm air escaping from the earth, and 

 of cold air descending from above, all tend to keep the 

 clouds in a state of agitation, upheaval, and depression. 

 Under their various influences the masses of vapor 

 composing the clouds are caused to assume all manner 

 of grotesque and fanciful shapes. 



545 What effect have winds on the shape of clouds f 



They sometimes absorb them entirely ; sometimes 

 increase their volume and density ; and sometimes 

 change the position of their parts. 



546 How can winds absorb clouds altogether ? 



Warm, dry winds will convert the substance of clouds 

 into invisible vapor, which they will carry away in 

 their own current. 



54*7 How can winds increase the bulk and density of clouds f 



Cold currents of wind w r ill condense the invisible 

 vapor of the air, and add it to the clouds with which, 

 they come in contact. 



54S Why is not the color of clouds always alike f 



Because their size, density, and situation in regard to 

 the sun are perpetually varying, so that sometimes one 

 color is reflected and sometimes another. 



549 Why do the clouds after sunset about the western horizon often 

 exhibit a beautiful crimson appearance ? 



Because the red rays, of which the sun's light is in 

 part composed, are less refrangible than any of the 

 other colors. In consequence of this, they are not bent 

 out of their course so much as the blue and yellow rays, 

 and are the last to disappear. 



For the same reason they are the first to appear in 



