the centre of the forehead--were imagined to be the workmen who
laboured in these underground forges. The noises, proceeding from
the heart of the mountain, were attributed to their operations. It
is to the Island of Hiera that Virgil alludes in the AEneid, lib.
viii. 416. The passage is thus rendered by Dryden:--

"Sacred to Vulcan's name, an isle there lay,
Betwixt Sicilia's coasts and Lipare,
Raised high on smoking rocks, and deep below,
In hollow caves the fires of Etna glow.
The Cyclops here their heavy hammers deal;
Loud strokes and hissings of tormented steel
Are heard around; the boiling waters roar,
And smoky flames through fuming tunnels soar."

A volcano generally presents itself to the imagination as a
mountain sending forth from its summit great clouds of smoke with
vast sheets of flame, and it is not unfrequently so described. The
truth is, however, that a real volcano seldom emits either true
smoke or true flame. What is mistaken for smoke consists merely of
vast volumes of fine dust, mingled with much steam and other
vapours--chiefly sulphurous. What appears like flames is simply the
glare from the glowing materials which are thrown up towards the
top of the mountain--this glare being reflected from the clouds of
dust and steam.

[Illustration: Peak of Teneriffe.]

The most essential part of a volcano is the crater, a hollow basin,
generally of a circular form. It is often of large dimensions, and