184 THE STORY-BOOK OF SCIENCE 



an illusion of light ; but under this illusion are con- 

 cealed the reservoirs of rain, source of the earth's 

 fecundity. God, by whom the smallest details of 

 creation have been ordered, willed that the most 

 common but also most necessary substances should 

 serve as an ornament to the earth in spite of their 

 really humble aspect; and he clothes them with a 

 prestige dependent on the distance from which we 



are to contemplate 

 them. The gray va- 

 por of the clouds 

 gives us rain. That 

 is its chief utility. 

 The sun illuminates 



it, and that suffices to transform it into a celestial 

 tapestry in which the astonished eye finds the splen- 

 dor of purple, gold and fire. That is its ornamental 

 function. 



"The height maintained by clouds is very vari- 

 able and is generally less than you might suppose. 

 There are clouds that lazily trail along the ground ; 

 they are the fogs. There are others that cling to the 

 sides of moderately high mountains, and still others 

 that crown the summits. The region where they are 

 commonly found is at a height varying from 500 to 

 1500 meters. In some rather rare instances they 

 rise to nearly four leagues. Beyond that eternal 

 serenity reigns; clouds never mount there, thunder 

 never rumbles, and snow, hail, and rain never form. 

 "Those clouds are called ' cirrus' that look some- 

 times like light flocks of curly wool, sometimes liko 

 drawn-out-filaments of dazzling whiteness, sharply 



