WIXDS. 



281 



escapes from the clouds in so short a time is very great, and masses have 

 been observed to fall of a weight of 10 or 12 ounces. 



Fig. 227. 



619. Hail-stones are generally pear-shaped, and if they are divided through 

 the center, they wiU be found to be composed of alternate layers of ice and 

 Bnovr, arranged around a nucleus, like the coats of an onion. 



Hail-stonns occur most frequently in temperate climates, and rarely within 

 the tropics. They occur most frequently in northern latitudes, in the vicinity 

 of high mountains, whose peaks are always covered with ice and snow. The 

 Eouth of France, which lies between the Alps and Pyrenees, is annually rav- 

 aged by hail ; and the damage which it causes yearly to vineyards and stand- 

 ing crops has been estimated at upward of nine millions of dollars. 



SECTION III. 



620. Wind is air put in motion. The air is 



■WhaUsWind? .-if?. • •. 



never entirely tree trom motion, but the ve- 

 locity with which it moves is perpetually varying. 



621. The principal cause of movements in 



What is the , i • 7i • • n 



principal cause tlic atmospliere is the variation of temperature 

 produced by the alternation of day and night 

 and the succession of the seasons. 



How can van- When, through the agency of the sun, a particular portion 



perXrf pro- °^ *^^® earth's surface is heated to a greater degree than the 

 duce wind f remainder, tho air resting upon it becomes rarefied and 



