THE STORY-BOOK OF SCIENCE 



comparison, representing the earth by an egg. Well, 

 the egg-shell is the solid crust of the globe; its 

 liquid content is the central mass in fusion. " 



"And we are separated from the immense subter- 

 ranean furnace only by that thin shell !" exclaimed 

 Jules. "That is not at all reassuring.'' 



"I agree, it is not without a certain emotion that 

 one hears for the first time what science tells us of 

 these intimate details of the earth's structure; one 

 cannot think without fear of those burning abysses 

 that roll their waves of melted minerals a few 

 leagues under our feet. How can a covering, rela- 

 tively so light, resist the fluctuations of the central 

 liquid mass? This fragile crust, this shell of the 

 globe, will it not some time melt, become disjointed, 

 crumble, or at least move? The little it does move 

 makes continents tremble and the ground crack open 

 in frightful chasms. ' ' 



"Ah!" interposed Claire, "that is the cause of 

 earthquakes. The liquid that is inside is stirred, 

 and the shell moves." 



"It seems to me," Jules remarked, "that this 

 shell, comparatively so thin, ought to tremble 

 oftener." 



"Perhaps not a day passes without the solid crust 

 of the earth experiencing some shock, sometimes at 

 one point, sometimes at another, beneath the bed 

 of the seas, as well as under the continents. How- 

 ever, disastrous earthquakes are very rare, thanks 

 to the intervention of volcanoes. 



"Volcanic orifices are, in fact, veritable safety- 

 valves, which put the interior of the globe in com- 



