Mr. A. Tylor on Tides and Waves. 217 



locity of the wave, instead of the impossible velocity of 800 miles 

 per hour suggested by some authors. 



An earthquake might transmit a blow through the deep water 

 in the ocean at six miles per minute, as a wave was formed at 

 Barbadoes 3000 miles from the supposed origin of the shock in 

 585 minutes after it was observed at Lisbon. Michell, in 1755 

 wrote " when the bar at the mouth of the Tagus was seen dry 

 from shore to shore, then suddenly the sea, like a mountain, came 

 rolling in." When this blow struck a distant coast below the level 

 of the sea, it would be reflected and cause the sea to ebb from the 

 coast first. Then when the force which heaped up the water away 

 from the shore diminished by work done upon the water in raising 

 up the level of the sea, a great wave would be moved shorewards 

 by gravity. The first announcement of the approach of an 

 earthquake- wave is the ebb of the water*, not a surface- wave. 



If a great surface-wave were generated by an earthquake, it 

 would not travel very far, but would soon diminish in height 

 and speed, and would not be preceded by a wave in an opposite 

 direction. 



In some careful experiments in a course of one fifth of a mile f 

 a surface- wave lost five sixths of its height. A powerful shock 

 or impulse could possibly be communicated through deep water, 

 like a blow through a solid body, an immense distance with 

 great velocity ; but that is not the case of a surface- wave at all. 



There is, therefore, a great distinction between primitive tidal 

 impulses and the secondary waves that accompany or follow 

 them, or the movements in coast-water produced at distant places 

 and times by means of the composition of forces. The tidal im- 

 pulse is communicated rather in the manner motion is conveyed 

 from a steam-engine through mechanical gearing, such as drivers 

 and followers, and where there is lost time and lost motion be- 

 tween the teeth of the driving-wheels, or bands and pulleys, or 

 levers, or other parts of the apparatus through which the move- 

 ment is communicated from a prime mover to some distant 

 point. 



Thus the piston may have commenced its down-stroke before 

 the effect of the former up- stroke had reached the extremity of 

 the shafting. This lost motion is very perceptible in figs. 1 and 

 2, Plate II., and fig. 1, Plate III. 



The particles of water may revolve along their axes ; or all the 

 vibrations may not be effective, some of them neutralizing others, 

 and for a short time destroying the impulse of the central tide- 

 generating force, soon to be renewed. 



The hours at which high water arrives are written against the 



* Michell, Phil. Trans. 1755. 

 t Brit. Assoc. 1838, p. 465. 



