INTRODUCTION. 9 
pebbles, rubs them against each other, polishes them, degrades and 
reduces them to fine sand, which is deposited upon the lowest areas 
of the ocean, or accumulates upon the beach. The most violent 
waves batter the submarine escarpments, and aid in splitting them 
into fragments; but, arrested and turned by the layers of water 
which cover them, the ascending currents become ground waves, 
which move with appalling rapidity, and break against the shore 
with irresistible force. During the tempest of 1822 the waves in 
the Bay of Biscay, at the rocks of Orta, were 1,200 feet long, and 
rolled with a velocity of sixty-six feet a second; that is, twice the 
velocity of a locomotive going at thirty miles an hour. According 
to Colonel Emy, the ground waves operate at a depth of 400 feet, 
and are capable of lifting the water above them in columns, more 
than 150 feet above the surface, heaving up a mass of water from 
20,000 to 30,000 cubic feet in volume, and weighing two or three 
thousand tons. 
These ground waves play a prominent part in many of the 
ocean phenomena. They are found in every sea; and it is they, 
and not the surface waves, which carry towards the shore the 
pebbles, sand, and dédris of shells and other submarine objects. 
They again, upon the submarine banks, produce those breakers so 
dreaded by sailors, and which render the navigation of certain bays 
impracticable even in the calmest weather. It is by these ground 
waves that that singular phenomenon is explained which is seen at 
the mouths of great rivers, called by the mariners of the Seine 
the barre; mascaret by the sailors of the Dordogne; and pororora 
by the boatmen of the Amazon. At the mouth of this last river, 
when it is spring-tide, the sea, instead of taking six hours to rise, 
attains its full height in two or three minutes. A wave fourteen or 
sixteen feet high, spreads itself over the whole breadth of the river. 
It is soon followed by two or three similar ones, and all rush up the 
stream with a deafening roar, and such a rapidity that they sweep 
away every obstacle in their course, uprooting trees and tearing 
away acres of the shore. The pororora affects the waters of the 
river for 600 miles into the interior. 
Another terrible whirlpool of the sea has been called the 
mistrém or midlstrém. It is a kind of permanent and eternal 
trough, which is to be seen in the North Sea, between Mosken and 
