PRIMER OF PRIMERS. 203 



which represent the largest forms of plant life, 

 should be preserved. 



Hail occurs when considerable differences of 

 temperature exist between neighboring masses of 

 very moist air. 



The destruction of the forest, by readily per- 

 mitting such differences of temperature to occur, 

 tends to increase the number and severity of hail- 

 storms. 



A hail-storm is generally preceded by the ap- 

 pearance of several layers of dark grayish clouds, 

 and a violent movement is often seen to occur be- 

 tween them, that is probably of a whirling char- 

 acter. 



Hail-storms are almost invariably attended by 

 marked disturbances in the electrical equilibrium 

 of the atmosphere. 



A hailstone is formed of alternate layers of ice 

 and snow. Various explanations have been offered 

 to account for the peculiar structure. Volta as- 

 cribed it to the alternate attractions and repulsions 

 occurring between neighboring snow- and rain- 

 clouds, when charged with opposite kinds of elec- 

 tricity. 



In France, where Volta's theories were formerly 

 received, lightning-rods were fruitlessly erected on 



