RESISTANCE TO VOLCANIC ACTION. 637 
of mass that man can pile up upon the surface. But the fact 
that in countries subject to earthquakes many very large and 
strongly constructed palaces, temples, and other monuments 
have stood for centuries, comparatively uninjured, suggests a 
doubt whether this opinion is sound. The earthquake of the 
first of November, 1755, which is asserted, though upon doubt- 
ful evidence, to have been felt over a twelfth part of the earth’s 
surface, was among the most violent of which we have any 
clear and distinct account, and it seems to have exerted its most 
destructive force at Lisbon. It has often been noticed as a re- 
markable fact, that the mint, a building of great solidity, was 
almost wholly unaffected by the shock which shattered every 
house and church in the city, and its escape from the common 
ruin can hardly be accounted for except upon the supposition 
that its weight, compactness, and strength of material enabled 
it to resist an agitation of the earth which overthrew all weaker. 
structures. On the other hand, a stone pier in the harbor of 
Lisbon, on which thousands of people had taken refuge, sank 
with its foundations to a great depth during the same earth- 
quake; and it is plain that where subterranean cavities exist, 
at moderate depths, the erection of heavy masses upon them 
would tend to promote the breaking down of the strata which 
roof them over. 
No physicist, I believe, has supposed that man can avert the 
eruption of a volcano or diminish the quantity of melted rock 
which it pours out of the bowels of the earth; but it is not 
always impossible to divert the course of even a large current 
of lava. “The smaller streams of lava near Catania,” says 
Ferrara, in describing the great eruption of 1669, “ were turned. 
from their course by building dry walls of stone as a barrier 
against them. . . . It was proposed to divert the main current 
from Catania, and fifty men, protected by hides, were sent with 
hooks and iron bars to break the flank of the stream near Bel- 
passo.* When the opening was made, fluid lava poured forth 
* Soon after the current issues from the volcano, it is covered above and at 
its sides, and finally in front, with scoriz, formed by the cooling of the ex- 
