f different density between the observer and the
horizon. Distant objects seem to be just under one's feet when seen from
the summit of the mountain. Smyth gives the radius of vision as 150.7
miles: and this we are inclined to adopt as the nearest approach to the
truth, because Smyth was an accurate observer, and he made careful
corrections both for error of instruments and for refraction. This
radius gives an horizon of 946.4 miles of circumference, and an
included area of 39,900 square miles--larger than the area of Ireland.
If a circle be traced with the crater of Etna as a centre, and a radius
of 150.7 miles, it will be found to take in the whole of Sicily and
Malta, to cut the western coast of Italy at Scalca in Calabria, leaving
the south-east coast near Cape Rizzuto. Such a circle will include the
whole of Ireland, or if we take Derby as the centre, its circumference
will touch the sea beyond Yarmouth on the East, the Isle of Wight on the
South, the Irish Channel on the West, and it will pass beyond Carlisle
and Newcastle-on-Tyne on the North.

The road which surrounds the mountain is carried along its lower slopes,
and is 87 miles in length. It passes through the towns of Paterno,
Aderno, Bronte, Randazzo, Linguaglossa, Giarre, and Aci Reale. It is
considered by some writers to define the base of the mountain, which is
hence most erroneously said to have a circumference of 87 miles; but the
road frequently passes over high beds of lava, and winds considerably.
It is about 10 miles from the crater on the North, East, and West sides,
increasing to 15-1/2 miles at Paterno, (S.W.). The elevation on the North
and West flanks of the mountain is nearly 2,500 feet, while on the South
it falls to 1,500 feet, and on the East to within 50 feet of the level
of the sea. It is quite clear that it cannot be asserted with any
degree of accuracy to define the base of the mountain.

[Illustration: Sections of Etna]

The "natural boundaries" of Etna are the rivers Alcantara and Simeto on
the North, West, and South, and the sea on the East to the extent of 23
miles of coast, along which lava streams have been traced, sometimes
forming headlands several hundred feet in height. The base of the
mountain, as defined by these natural boundaries, is said to have a
circumference of "at least 120 miles," an examination of the new map,
however, proves that this is over-estimated.

If we take the sea as the eastern boundary, the river Alc