428 



F. A. Ferret— Vt 



esuvius. 



horizontally and then descend in graceful curves, while the 

 bulk of the avalanche, enveloped in whirling clouds, falls from 

 precipice to precipice with the reverberating roar of thunder 

 until it finally precipitates itself upon a talus at the bottom of 

 the crater. Then ensues the development of a magnificent 

 dust-cloud, flaring and torch-like at first, but it soon f orms a com 

 pact cauliflower cloud of exquisite beauty, reminding one irresist- 

 ibly of the "nuees ardentes".* The motion, both of transla- 



Fig. 12. 



Fig. 



crater 



12. Vesuvius— Development of an avalanche on reaching bottom of 



tion and of development, is exceedingly rapid, and the cloud 

 unfolds and advances with sharply defined contours. It should 

 be noted that we have here no vapor of water, no high tem- 

 perature and little, if any, electrical potential — nothing, that is 

 to say, which could constitute anything like an "emulsion" and 

 it would seem that all that is required for producing sharp 

 outlines in a dust-cloud is sufficiently rapid projection against 

 an air-cushion. 



The smaller of these avalanches are interesting as showing 

 more clearly the various phases of their development, the 

 wind often blowing to one side the lighter dust, where it may 



*Lacroix, La Montagne Pele'e, Paris, 1904. 



