

Parr Il. Sect. ii. § 6.] MARINE EROSION. 429 
the walls were breached by a severe bombardment. At the Eddystone 
lighthouse, during a storm in 1840, a door which had been securely 
fastened against the force of the surf from without, was actually 
driven outward by a pressure acting from within the tower, in spite 
of the strong bolts and hinges, which were broken. We may infer 
that, by the sudden sinking of a mass of water hurled against the 
building, a partial vacuum was formed, and that the air inside forced 
out the door in its efforts to restore the equilibrium. ‘This explana- 
tion may partly account for the way in which the stones are started 
from their places in a solidly built sea-wall. But besides this cause 
we must also consider a perhaps still more effective one in the con- 
densation of the air driven before the wave between the joints and 
crevices of the stones, and its subsequent instantaneous expansion 
when the wave drops. During gales, when large waves are driven’ 
to shore, many tons of water are poured suddenly into a cleft or 
cavern. ‘These volumes of water, as they rush in, compress the 
air into every joint and pore of the rock at the further end, 
and then quickly retiring, exert such a suction as from time to time 
to bring down part of the walls or roof. The sea may thus gradually 
form an inland passage for itself to the surface above, in a “blow- 
hole ” or “ puffing-hole,” through which spouts of foam and spray are 
in storms shot high into the air. On the more exposed portions of 
the west coast of Ireland numerous examples of such blow-holes occur. 
In Scotland, likewise, they may often be observed, as in the Bullers 
(boilers) of Buchan on the coast of Aberdeenshire, and the Geary Pot 
near Arbroath. Magnificent instances occur among the Orkney and 
Shetland Islands, some of the more shattered rocks of these northern 
coasts being, as it were, honeycombed by sea-tunnels, many of which 
open up into the middle of fields or moors. 
y. The hydraulic pressure of those portions of large waves 
that enter fissures and passages tends to force asunder masses of rock. 
The sea-water which, as part of an in-rushing wave, fills the gullies 
and chinks of the shore-rocks exerts the same pressure upon the walls 
between which it is confined as the rest of the wave is doing upon the 
face of the cliff. Hach cleft so circumstanced becomes a kind of 
hydraulic press, the potency of which is to be measured by the force 
with which the waves fall upon the rocks outside—a force which 
often amounts to three tons on the square foot. There can be little 
doubt that by this means considerable pieces of a cliff are from time 
to time dislodged. 
6. The waves make use of the loose detritus within their 
reach to break down cliffs exposed to their fury. Probably by far 
the largest amount of erosion is thus accomplished. The blows 
dealt against shore-cliffs by boulders, gravel, and sand swung forward 
by breakers, were aptly compared by Playfair to a kind of artillery.’ 
During a storm upon a shingly coast we may hear, at a distance of 
1 Walker, Proc. Inst. Civ. Engin. i. p. 15; Stevenson’s “ Harbours,” p. 10. 
2 * Vilustrations of the Hutionian Theory,” sec. 97. 
