652 WILLIAM HERBERT HOBBS 



the great uprush of the steam. In times of grand eruption Vesuvius 

 becomes a gigantic steam injector (see Fig. 3). As already stated, 

 the light wind blew at the time of the grand paroxysm from the 

 southwest, or over the crater toward Ottaiano. On the side toward 

 the bay the force of this current would simply be increased, whereas 

 on the Ottaiano side the southwest wind would be replaced near the 

 surface by a contrary current blowing in the direction of the crater. 

 From the fact that lapilli even are not found west of the crater at 

 points below the piano, it is clear that projectiles which fell at Ottaiano 

 must have followed a course determined largely by air currents, 

 and presumably similar to that indicated in the diagram. That 

 they reached the ground in directions inclined at fairly flat angles 

 in the direction of the volcano would appear to follow from the damage 

 done to the upper portion of windows protected by casings nearly 

 or quite a foot in depth. 



The ash layer to the north and west. — To the west of Somma, 

 and along the shores of the bay only ash is found. Its depth may 

 reach six inches between Portici and Torre del Greco, though else- 

 where it is generally considerably less. Higher up on the slopes 

 its thickness increases until at the observatory it attains a thickness 

 of perhaps ten inches, and with it are associated extremely fine lapilli. 



In Naples the depth of the ash deposit hardly exceeded an inch, 

 certainly not an inch and a half. The collapse of the roof of the 

 market of Monte Oliveto, and of a few other buildings in Naples, 

 is adequately explained by the high specific gravity of the ash and 

 the weak construction of the buildings. The agriculture of all 

 this northwestern, western, and southern belt will quickly recover 

 from the loss which it has sustained. As regards Somma, Ottaiano, 

 San Giuseppe, and to a less extent the more distant cities to the 

 northeast of the mountain, a considerable time must probably elapse 

 before the soil will again become productive. 



The truncation 0} the cone. — It is impossible at this writing to 

 give accurate figures concerning the reduction in height of Vesu- 

 vius; but that the cone has been reduced to near the level of the 

 Punta del Nasone, the highest point of Somma, appears from views 

 of the mountain.' It was reported in the newspapers, upon the 



I Such data are illusory and later measurements show the truncation to have been 

 much less. 



