THUNDER. 9 



A. When the rent air closes again, 

 it is said to collapse. 



Q. Why is the PEAL sometimes an IRREGULAR 

 mangling broken ROAR ? 



A. When the lightning-cloud is a 

 long way off, as the flash is zigzag, the 

 air does not collapse all at once ; and as 

 we hear the concussion of one part after 

 another, the peal is broken, protracted, 

 and irregular. 



Q. Which part of the collapsing air do we 

 hear first ? 



A. That part nearest the earth; then 

 the strata above ; and last of all, that in 

 the immediate vicinity of the cloud. 

 Q. What is meant by "STRATA of air ?" 

 A. If a board were laid upon the 

 earth, and several other boards were piled 

 upon it, this pile would represent strata 

 of wood. 



Q. How does this illustration apply to the 

 air? 



A. A layer of air covers the earth ; 

 another layer rests upon it; and thus 

 layer is piled upon layer, for 50 miles in 

 height. Each layer is a " stratum" of 

 air ; and the plural of stratum is strata. 



