continues to flow
in the under arches. In this state of the movement trifling accidents,
or even human interference, may direct the current this way or that.

Some of the most interesting chapters in the history of AEtna relate to
the efforts of the people to turn these slow-moving streams so that
their torrents might flow into wilderness places rather than over the
fields and towns. In the great flow of 1669, which menaced the city of
Catania, a large place on the seashore to the southeast of the cone, a
public-spirited citizen, Senor Papallardo, protecting himself and his
servants with clothing made of hides, and with large shields, set
forth armed with great hooks with the purpose of diverting the course
of the lava mass. He succeeded in pulling away the stones on the
flank of the stream, so that a flow of the molten rock was turned in
another direction. The expedient would probably have been successful
if he had been allowed to continue his labours; but the inhabitants of
a neighbouring village, which was threatened by the off-shooting
current which Papallardo had created, took up arms and drove him and
his retainers away. The flow continued until it reached Catania. The
people made haste to build the city walls on the side of danger higher
than it was before, but the tide mounted over its summit.

Although the lavas which come forth from the volcano evidently have a
high temperature, their capacity for melting other rocks is relatively
small. They scour these rocks, because of their weight, even more
energetically than do powerful torrents of water, but they are
relatively ineffective in melting stone. On AEtna and elsewhere we may
often observe lavas which have flowed through forests. When the tide
of molten rock has passed by, the trees may be found charred but not
entirely burned away; even stems a few inches in diameter retain
strength enough to uphold considerable fringes and clots of the lava
which has clung to them. These facts bear out the conclusion that the
fluidity of the heated stone depends in considerable measure on the
water which is contained, either in its fluid or vaporous state,
between the particles of the material.

If we consider the Italian volcanoes as a whole, we find that they lie
in a long, discontinuous line extending from the northern part of the
valley of the Po, within sight of the Alps, to AEtna, and in
subterranean cones perhaps to the northern coast of Africa. At the
northern e