
Parr Il. Szcr. ii. §6.] | SEA-WAVES. 493 
water mark, are said to be sometimes broken by stones swept up the 
cliffs by the sheets of sea-water which then deluge the building. 
A single roller of the ground-swell 20 feet high falls, according 
to Mr. Scott Russell, with a pressure of about a ton on every square 
foot. Mr. Thomas Stevenson conducted some years ago a series of 
experiments on the force of the breakers on the Atlantic and North 
Sea coasts of Britain. The average force in summer was found in 
the Atlantic to be 611 Ib. per square foot, while in winter it was 
2086 lb., or more than three times as great. But on several occa- 
sions, both in the Atlantic and North Sea, the winter breakers were 
found to exert a pressure of three tons per square foot, and at 
Dunbar as much as three tons and a halft Besides the waves 
produced by ordinary wind action, others of an extraordinary size and 
destructive power are occasionally caused by a violent cyclone-storm. 
The mere diminution of atmospheric pressure in a cyclone must tend 
to raise the level of the ocean within the cyclone limits. But the 
further furious spiral in-rushing of the air towards the centre of the 
low pressure area drives the sea onward, and gives rise to a wave or 
succession of waves having great destructive power. Thus, on 5th 
October, 1864, during a great cyclone which passed over Calcutta, 
the sea rose in some places 24 feet, and swept everything before 
it with irresistible force, drowning upwards of 48,000 people. 
Besides the height and force of waves it is important to know 
the depth to which the sea is affected by such superficial movements. 
The Astronomer-Royal states that ground-swell may break in 100 
fathoms water.” It is common to find boulders and shingle dis- 
turbed at a depth of 10 fathoms, and even driven from that depth 
to the shore, and waves may be noticed to become muddy from the 
working up of the silt at the bottom when they have reached water 
of 7 or 8 fathoms in depth. Itis stated by Delesse that en- 
gineering operations have shown that submarine constructions are 
scarcely disturbed at a greater depth than 5 metres(16-4 feet) in 
‘the Mediterranean and 8 metres (26°24 feet) in the Atlantic.* In the 
Bay of Gascony it has been ascertained that the depth at which the 
sea breaks and is effective in the transport of sand along the bottom 
varies from scarcely 3 metres in ordinary weather, to 5 metres in 
stormy weather, and only exceeds 10 metres (82°8 feet) in great 
hurricanes. According to Commander Cialdi, the movement of 
waves may disturb fine sand on the bottom at a depth of 40 metres 
(131 feet) in the English Channel, 50 metres (164 feet) in the 
Mediterranean, and 200 metres (656 feet) in the ocean.® 
(4.) Ice on the Sea.—In this place may be most conveniently 
1 T. Stevenson, Trans. Roy. Soe. Edin. xvi. p. 25; treatise on “ Harbours,” p. 42. 
2 Encyclopedia Metropolitana, art. ‘‘ Waves.” Gentle movement of the bottom water 
is said to be sometimes indicated by ripple-marks on the fine sand of the sea-floor at a 
depth of 600 feet. 
3 T. Stevenson’s “ Harbours,” p. 15. 
4 «“Lithologie des Mers de France ” (1872), p. 110. 
> Quoted by Delesse, op. cit. p. 11]. 
