156 TIIK STORY-BOOK OF SCIENCE 



the sky. If the vault broke anywhere, the ball was 

 dashed to the ground and the thunder fell. 

 But I don't believe that now. I am too big." 



"Too big a little fellow not so high as the first 

 button on my vest ! Say rather that your little rea- 

 soning powers are awakening and that the simple ex- 

 planation of the iron ball no longer satisfies them." 



Then Claire spoke. "I am not satisfied either 

 with the explanations I used to give myself a while 

 ago. With me, thunder was a wagon heavily loaded 

 with old iron. It rolled on top of a sonorous vault. 

 Sometimes a spark would flash out from under the 

 wheels, the same as from a horse's hoof when it 

 strikes a stone: that was the lightning. The vault 

 was slippery and bordered with precipices. If the 

 wagon happened to tip over, the load of old iron 

 would fall to the ground, crushing people, trees, and 

 houses. I laughed yesterday at my explanation, but 

 I am no farther advanced now : I still know nothing 

 at all about thunder." 



"Your two thunders, varying to suit your infant 

 imaginations, are based on the same idea, the idea of 

 a sonorous vault. Well, know once for all that the 

 blue vault of the sky is only an appearance due to the 

 air which envelops us, and which, owing to the thick- 

 ness of the envelope, has a beautiful blue color. 

 Around us there is no vault, only a thick layer of 

 air ; and beyond that there is nothing for a vast dis- 

 tance until you come to the region of the stars. )r 



"We will give up the blue vault," said Jules. 

 "Emile, Claire, and I are persuaded there is n't any. 

 Please go on." 



