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 been accurately investigated by the late Rev. Dr. Scoresby, during two passages across the Atlantic in 1847 and 1848. "In the afternoon of March 5th, 1848," says that eminent philosopher, "I stood during a hard gale upon the cuddy-roof or saloon deck of the 'Hibernia:' a height, with the addition of that of the eye, of 23 feet 3 inches above the line of flotation (the ship's course being similar to that of the waves). I am not aware that I ever saw the sea more terribly magnificent; the great majority of the rolling masses of water was more than 24 feet high, (including depression as well as altitude, or reckoning above the mean-level, more than 12 feet). I then went to the larboard paddle-box, about 7 feet higher (30 feet 2 inches up to the eye), and found that one half of the waves rose above the level of the view obtained. "Frequently I observed long ranges (200 yards), which rose so high above the visible horizon, as to form an angle estimated at two or three degrees when the distance of the wave's summit was about 100 yards from the observer. This would add near 13 feet to the level of the eye, and at least one in half-a-dozen waves attained this altitude. Sometimes peaks or crests of breaking seas would shoot upward, at least 10 or 15 feet higher.