' WATER AS A MECHANICAL AGENT. 213 



attendant waves, are the efficient agents, because they act directly against 

 and along the coasts, and have great power. 



Storm-winds, as stated on page 159, have often a velocity of 60 to 100 

 miles an hour. They have built up, by drift-sands alone, the east side of the 

 Bermuda reefs to a height exceeding 200 feet, while the regular winds have 

 not raised the side of the coral reef facing them above high-tide level. They 

 have made similar drift-hills on the Bahamas, and over the Florida reefs. 



Waves rise in long lines transverse to the course of the winds, but with 

 irregularities in the lines, owing to veerings and other variables in the driv- 

 ing agent. Their height depends on the size of the sea, as well as on the 

 winds, and in shallow water on its depth. But every seventh or eighth wave 

 is often a maximum, it being a combination of two, one overtaking another. 



Waves have at times great height. The highest measured by Scoresby 

 stood 43 feet above the intervening trough, or 21^ feet above the mean 

 water-plane or plane of rest. According to results obtained by the United 

 States Hydrographical Department, the storm-waves of the North Atlantic 

 have a maximum height of 44 to 48 feet, but ordinarily a height of 30 feet, 

 and a length of 500 to 600 feet. 



But the depth of the action of waves is moderate. In a wave, each par- 

 ticle of water moves in a circle about its center of rest, — a circle of 21^ 

 feet radius in a wave of 43 feet. But these circles at a depth of only one 

 wave-length have a radius -^^-^ of that at the surface, and at a depth of tiuo 

 wave-lengths,' 3 ^ q^q ^ ^ ; so that if, for the 43-foot waves, the wave-length — 

 or the distance between the crest of two consecutive waves — is 300 feet, 

 the circle at a depth of owe wave-length will have a diameter of -^ of an 

 inch, and at two wave-lengths, yxo "o ^^ ^^^ inch. Consecxviently the move- 

 ment of the heaviest waves in the open ocean is exceedingly slight, if appar- 

 ent at all, at a depth of 100 fathoms. This depth is the probable limit of 

 the movement of sand by wave-action, but not the limit of the action of 

 currents. 



3. Earthquake "Waves. 



In an earthquake, the movement of the earth may be either (1) a simple 

 vibration of a part of the earth's crust, or (2) a vibration with actual eleva- 

 tion or subsidence. If submarine waves are produced, they have a forward 

 impulse, and, in the second case, an actual forward movement or amplitude 

 equivalent to the amount of change of level ; in each case, therefore, they 

 are translation leaves. The velocity of propagation varies as the square root 

 of the depth, the number of miles per hour being 12-2 miles in a depth of 10 

 feet; 38-7 in that of 100 feet; 122-3 in that of 1000. An earthquake at 

 Concepcion, Chile, set in motion a wave that traversed the ocean to the 

 Society and Navigator Islands, 3000 and 4000 miles distant, and to the 

 Hawaiian Islands, 6000 miles ; and on Hawaii it swept up the coast, tem- 

 porarily deluging the village of Hilo. An earthquake at Arica, and other 

 parts of southern Peru, August 14, 1868, sent a wave across the Pacific, west- 



