544 
RESISTANCE TO VOLCANIC ACTION. 
escape from tlie common ruin can hardly be accounted for ex¬ 
cept 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 earthquake ; and it is plain that 
where subterranean cavities exist, at moderate depths, the erec¬ 
tion 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 cur¬ 
rent 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 Belpasso.* When the opening was made, 
* Soon after the current issues from the volcano, it is covered above 
and at its sides, and finally in front, with scoriae, formed by the cooling of 
the exposed surface, which bury and conceal the fluid mass. The stream 
rolls on under the coating, and between the walls of scoriae, and it was the 
lateral crust which was broken through by the workmen mentioned in 
the text. 
The distance to which lava flows, before its surface begins to solidify, 
depends on its volume, its composition, its temperature and that of the air, 
the force with which it is ejected, and the inclination of the declivity over 
which it runs. In most cases it is difficult to approach the current at points 
where it is still entirely fluid, and hence opportunities of observing it in 
that condition are not very frequent. In the eruption of February, 1851, 
on the east side of Vesuvius, I went quite up to one of the outlets. The 
lava shot out of the orifice upward with great velocity, like the water 
from a spring, in a stream eight or ten feet in diameter, throwing up occa¬ 
sionally volcanic bombs, but it immediately spread out on the declivity 
down which it flowed, to the width of several yards. It continued red hot 
in broad daylight, and without a particle of scoriae on its surface, for a 
