F. A. Per ret — Vesuvius. 



427 



forms one of the most impressive sights which can be imagined. 

 Detachment sometimes takes place silently but more often with 



Fig. 11. 



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Fig. 11. Vesuvius — Descent of an avalanche inside the crater. 



a sharp crack. The acceleration is almost equal to that of a 

 freely falling body, as the crater walls are nearly perpendicular. 

 Huge bowlders, rebounding from the sills of lava, are projected 



