408 



F. A. ferret — Volcanic Vortex Rings. 



was five meters per second and the mass as a whole showed all 

 the qualities of a liquid, the viscosity was so great, especially 

 in the outer layer, that a heavy rock thrown upon the surface 

 rehonnded as from a plate of steel, and it was only with the 

 greatest difficulty that an iron rod could he forced into the 

 moving stream. In the very liquid lavas of Hawaii the gas 

 vesicles issue almost without resistance and do not form ash in 

 this way, hut large gas bubbles scatter the lava and spin it out 

 into glassy filaments — the well known " Pole's Hair." 



The present writer has often observed the same copper- 

 colored cloud of gas and ash as produced directly from glowing 

 lava in the central crater of Etna, and also in that of Vesuvius 



Fig. 3. 



1 ii |fl 



aaB 



■ 



i 







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| 





1 







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and Stromboli. A cloud of white vapor over the crater will also, 

 even in daylight, under these circumstances, appear of a cop- 

 per tint because illuminated by the glare of the lava below, 

 but when ash is present the color of the cloud persists to all 

 distances, as in the case of the Etna rings. 



Let us now leave the consideration of the principle as exem- 

 plified in this superficial action, and study its function in the 

 more fundamental processes of a great eruption. Taking the 

 Vesuvian outbreaks of 1872 and 1906 as typical, and glancing 

 at conditions at the outset, we find that as the result of a long 

 period of moderate activity the cone has been built up to a 

 considerable elevation above the crater of the preceding great 

 eruption, and that the lava stands at a commensurate!} 7 high 



