662 DYNAMICAL GEOLOGY. 



in the surface-waters ; and, when the direction is reversed for half the 

 year, as in the western half of the tropical Pacific, the current is 

 changed accordingly. These currents become marked along shores, 

 and especially through open channels ; the great currents of the ocean 

 are attributed by some physicists to the force of the prevailing winds. 

 Prolonged storms often produce their own currents, even in mid-ocean, 

 and more strikingly still among the bays and inlets of a coast. 



These currents made by the winds are inferior in power to the tidal 

 currents, among the inlets and islands of a continental coast ; but, 

 about oceanic islands, they are often of greater strength. 



8. Undercurrents. — The forcing of waters into bays, whether by 

 regular winds or by storms, causes a strong under-current outward, like 

 that from the tides. This happens when the entrance of the bay is 

 broad, so as to allow of an in-flow over a wide area, while the deep- 

 water channel is narrow, and especially so, if the entrance to the bay 

 is narrowed by a bar or reef. In some cases, ships lying at anchor 

 feel this under-current so strongly as to " tail out " the harbor, in the 

 face of a gale which is blowing in. 



In the ordinary breaking of waves on a beach or in rocky coves, 

 there is an under-current (or under-tow) flowing outward along the 

 bottom. The wave advances and makes its plunge ; and then its 

 waters flow back beneath those of the next wave, which is already 

 hastening on toward the beach. 



4. 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 elevation or subsidence. In each case, the ocean-waves, which 

 the earthquake, if submarine, may produce, have an actual forward im- 

 pulse, and are, therefore, forced or translation-waves. They have great 

 power ; and, as there is no narrow limit to the amount of elevation 

 which may attend an earthquake, such a wave may be of enormous 

 height. An earthquake at Concepcion, Chili, set in motion a wave 

 that traversed the ocean to the Society and Navigator Islands, 3,000 

 and 4,000 miles distant, and to the Hawaian Islands, 6,000 miles ; and 

 on Hawaii it swept up the coast, temporarily deluging the village of 

 Hilo. An earthquake at Arica, and other parts of southern Peru, 

 August 14, 1868, sent a wave across the Pacific, westward to New 

 Zealand and Australia, northwestward to the Hawaian Islands, north- 

 ward to the coast of Oregon. 



