26 PHYSICAL GEOGRAPHY OF TIE SEA. 
formation of small waves. Numberless oscillations unite their 
efforts, and create visible elevations and depressions. Mean- 
while, the wind is constantly setting new particles in motion; 
long before the first oscillations have lost their effect, countless 
others are perpetually arising, and thus the sum of the pro- 
pelling powers is constantly increasing, and gradually raising 
mountain-waves, until their growth is finally limited by the 
counterbalancing power of the earth’s attraction. 
As the strength of the waves only gradually rises, it also loses 
itself only by degrees, and many hours after the tornado has 
ceased to rage, mighty billows continue to remind the mariner 
of its extinguished fury. The turmoil of waters awakened by 
the storm propagates itself hundreds of miles beyond the space 
where its howling voice was heard, and often, during the most 
tranquil weather, the agitated sea proclaims the distant war of 
the elements. 
The velocity of waves depends not only on the power of the 
impulse, but also on the depth of the subjacent waters, as I have 
already mentioned in the preceding chapter. 
For this reason, as increased velocity augments the power of 
the impulse, the waves in the Atlantic or Pacific, the mean 
depth of which may be estimated at 12,000 or 18,000 feet, 
attain a much greater height than in the comparatively shallow 
North Sea. 
The breaking of the waves against the shore arises from their 
velocity diminishing with their depth. As the small flat wave 
rolls up the beach, its front part, retarded by the friction of 
the ground, is soon overtaken by its back, moving in swifter 
progression, and thus arises its graceful swelling, the toppling 
of its snow-white crest, and finally its pleasant prattle among 
the shingles of the strand. This is one of those pictures of 
nature which Homer describes with such inimitable truth in 
various places of his immortal poems: he paints with admirable 
colours the slow rising of the advancing wave, how it bends 
forward with a graceful curve, and, crowning itself with a 
diadem of foam, spreads like a white veil over the beach, 
leaving sea-weeds and shells behind, as it rustles back again 
into the sea. 
The height which waves may attain on the open sea has 
