672 DYNAMICAL GEOLOGY. 



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. 



3. The Tidal Wave and Tidal Currents. 



The tidal wave differs from ordinary waves in many respects ; (1) in 

 having an extra-terrestrial origin — the attraction of the moon and 

 sun — owing to which, the ocean feels the impulse to its bottom, and 

 the wave is a translation-wave ; (2) in the movement being westward 

 as a consequence of the earth's eastward revolution, and hence in 

 having the same rate of movement as the earth, or 1,000 miles an 

 hour at the equator (that is, movement in wave motion, not in water) ; 

 consequently (3) in having for the length of a single wave 12,000 

 miles, the ebb and flow occupying together twelve hours. The Pacific 

 is too narrow from east to west to contain at once much over half of 

 the wave-curve, and the North Atlantic could hold transversely but a 

 quarter of it. After leaving the Pacific, its course is northwesterly 

 in the Indian Ocean, and the same also in the Atlantic. The height 

 in the middle Atlantic is very small, but as the depth diminishes on 

 soundings, the wave increases in elevation, and its translation charac- 

 ter becomes more and more appreciable. Still, at the prominent head- 

 lands of the continent its height is only 1 to 2 feet ; being but 2 feet 

 at Cape Hatteras after traversing 80 miles of soundings, the outer 

 and deepest part of which is 100 fathoms. 



The height of the tide is augmented by converging coast-lines. 

 The eastern coast of North America has, as laid down by Bache, a 

 great " Southern Bay," between Florida and Cape Hatteras, a " Mid- 

 dle Bay " between Hatteras and Nantucket, and an " Eastern Bay," 

 north of Nantucket ; and, while at Southern Florida the tide is 1 to 

 \\ feet, at Cape Hatteras 2 feet, at Southeastern Nantucket only 1 

 foot, the height within the " Southern Bay," at Savannah, is 7 feet, in 

 the " Middle," at the entrance of New York Bay, 5 feet, and in the 

 "Eastern," at Boston, 10 feet; and in the narrow Bay of Fundy, 40 

 feet, and sometimes 70 feet at the spring tides. 



Other examples of unusual tide are the following : At the entrance to the British 

 Channel, England, the spring tides are 18 feet, and within it, at. the mouth of the Sev- 

 ern, 45 to 50 feet ; at the Bay of St. Michel (west coast of Normandy), France, 45 to 48 

 feet ; at the extremity of the Persian Gulf, 36 feet; while in the Gulf of Mexico, at the 

 mouth of the Mississippi, it is only 2 feet; and in the Mediterranean, because of its 

 little size, no tide is appreciable. 



