28 PHYSICAL GEOGRAPHY OF THE SEA. 
Thus Captain Wilkes, commander of the U. 8. Exploring 
Expedition, found the height of the waves near Orange Har- 
bour, where they rose higher and more regular than at any 
other time during the cruise, to be thirty-two feet (depression 
ana altitude), and their apparent progressive motion about 
twenty-six and a half miles in an hour. 
Sir James Ross calculated the height of the waves on a strongly 
agitated sea at twenty-two feet, and, according to the French 
naturalists who sailed in the frigate “ La Venus,” on her voyage 
round the world, the highest waves they met with never exceeded 
that measure. 
Thus, according to the joint testimony of the most eminent 
nautical authorities, the waves in the open sea never attain the 
mountain-height ascribed to them by the exuberant fancy of 
poets or exaggerating travellers. | But when the tempest surge 
beats against steep crags or rocky coasts it rises to a much 
more considerable height. The lighthouse of Bell Rock, though 
112 feet high, is literally buried in foam and spray to the very 
top during ground-swells, even when there is no wind. On the 
20th November, 1827, the spray rose to the height of 117 feet 
above the foundation or low-water mark, which, deducting 
eleven feet for the tide that day, leaves 106 feet for the height 
of the wave. The strength of that remarkable edifice may be 
estimated from the fact, that the power of such a giant billow 
13 equivalent to a pressure of three tons per square foot. 
In the Shetland Islands, which are continually exposed to the 
full fury of the Atlantic surge (for no land intervenes between 
their western shores and America), every year witnesses the 
removal of huge blocks of stone from their native beds by the 
terrific action of the waves. “In the winter of 1802,” says 
Dr. Hibbert, in his description of that northern archipelago, “a 
tabular-shaped mass, eight feet two inches by seven feet, was 
dislodged from its bed and removed to a distance of from 
eighty to ninety feet. I measured the recent bed from which a 
block had been carried away the preceding winter (A.p. 1818), 
and found it to be seventeen feet and a half by seven feet, and 
the depth two feet eight inches. The removed mass had been 
borne to a distance of thirty feet, when it was shivered into 
thirteen or more lesser fragments, some of which were carried 
