210 
ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
or below the Atrio del Cavallo, A, taken at 2000 feet above the sea-level, has been 
Fig. 13. 
\f 
removed and replaced since the year 79 a.d. This is certainly above the truth, and 
exceeds Professor Phillips’s estimate (Vesuvius, pp. 248-9 & c.) ; for it takes no account 
of the mass of Somma, S, and the remains of the old mountain at the sea side, H (Peda- 
mentina), still remaining, and lets these go against all the lava-streams since a.d. 79 
and the wind-diffused dust, the united volume of which cannot but be much less than 
that of Somma alone from the Punto del Naso down to the level of the Ilermitao-e. 
The base-angle of the imagined cone S t IT we take from Scrope, and the writer’s own 
observation of the angle of Somma at the level of the Atrio, at 24°, the total height of 
the Vesuvian summit t at 4250 feet, which gives for the altitude of the cone (removed more 
than once) 2250 feet and the diameter of base 10,400 feet, or for simplicity let us call it 
10,560 feet or 2 miles. The altitude being 0-426 
of a mile, we have 0‘ 446 cubic mile for the volume 
of the cone. Let it be assumed that this volume 
has been thrice blown away and evolved again 
during the last 1800 years; then 1-338 cubic 
mile of melted or heated and elevated material ^ . jo^oo Sf.-- >. 
represents the useful work expended on the vol- 
cano for the above period. 
196. Supposing that work not to be paroxysmal (as it really is in every volcano) but 
1 *338 
spread uniformly over the 18 centuries, we have = 0-000743 cubic mile per annum, 
or 109,368,114 cubic feet per annum, raised 2000 feet from sea-level to the base of the 
cone and ^ of 2250 feet further to its centre of gravity. 
Taking, as before, the weight of the material at °f a ton per cubic foot, we have 
for the lifting work 14,010,053,610 foot-tons, equivalent to 4,831,052,974 British units 
of heat. Dividing this by the units of heat in a cubic foot of mean crushed rock as 
before, we obtain 746,481 cubic feet, or less than - 20 0 * 0 0 0 of a cubic mile of crushed rock 
as necessary to do the annual lifting work of the cone itself. Allowing, as before, that 
double the number of units of heat are wasted of those usefully employed in lifting- 
work, we have for the lifting work and waste together *000015 cubic mile of mean 
