xxvin CAUSE OF THUNDER PEAL 77 



burst out at once. In proof of this, bear in mind 4 

 that the wind blowing through a tree, which cuts 

 it, hisses but does not roar. A broad blow, so to 

 speak, that dissipates the whole mass at once, is 

 required in order to the emission of a sound such 

 as is heard when there is thunder. 



XXIX 



MOREOVER, the atmosphere is by constitution 

 adapted to the transmission of sound. 1 Of necessity 

 this is so, since sound is nothing but an impact of 

 the atmosphere. The clouds that [as indicated] 

 are completely rent must therefore be hollow and 

 taut. One sees how much more resonant empty 

 vessels are than full, and distended ones than slack. 

 So this accounts for the sound of tambourines and 

 cymbals ; the former resound because the blow 

 upon the air is resisted at the farther side ; the 

 latter are beaten against the air directly, but unless 

 there were a cavity in the instrument it would not 

 tinkle. 



XXX 



SOME authors, including Asclepiodotus, are convinced 

 that thunder and lightning may also be produced 

 by the collision of certain solid bodies. Once Etna 

 was in violent eruption and cast up a huge quantity 

 of burning sand. The daylight was veiled with the 

 cloud of dust, and sudden night terrified the world. 

 On that occasion, they allege, there was much 

 thunder and lightning, produced, they maintain, 



1 The specific word vox = voice is used in the text. 



