THE TIDES. 



different at different places. In latitude 60 south, where there 

 is no interruption from land (except the narrow promontory of 

 Patagonia), the tide-wave will complete a revolution in a lunar 

 day, and travel at the rate of five hundred miles an hour. 

 On examining Dr. WhewelFs map of eotidal lines, it will be 

 seen that the great tide-wave from the Southern Ocean travels 

 from the Cape of Good Hope to the Azores in about twelve hours, 

 and from the Azores to the southernmost part of Ireland in about 

 three hours more. In the Atlantic, the hourly velocity in some 

 cases appears to be 10 of latitude, or near 700 miles, which is 

 almost equal to the velocity of sound through the air. From 

 the south point of Ireland to the north point of Scotland, the time 

 is eight hours, and the velocity about 160 miles an hour along 

 the shore. On the eastern coast of Britain, and in shallower 

 water, the velocity is less. From Buchanness to Sunderland it is 

 about sixty miles an hour ; from Scarborough to Cromer, thirty- 

 five miles ; from the North Foreland to London, thirty miles ; 

 from London to Richmond, thirteen miles an hour in that part of 

 the river. (Whewell, Phil. Trans. 1833 and 1836.) When we 

 speak of the velocity of the tidal wave, it must not be imagined 

 that the mass of water of which the wave is composed has this 

 velocity. If such were the case, its momentum would be 

 destructive indeed. The motion of the tidal wave is only a 

 particular instance of undulatory motion, which is so often mis- 

 understood, and so frequently imputed to the fluid on which the 

 wave is formed, that it may be worth while here to explain it in 

 general. 



15. When we see the waves, produced on the surface of the 

 deep, apparently moving in a certain direction, we are very 

 naturally impressed, in the first instance, with the notion that the 

 sea itself is moving in that direction. We imagine that the same 

 wave, as it advances, is composed of the same water, and that the 

 whole surface of the liquid is in a state of progressive motion. 

 The least reflection, however, on the consequences of such a 

 supposition, will soon convince us that it is unfounded. The ship 

 which floats upon the sea, is not carried forward with the waves. 

 They pass in succession under her, now lifting her on their 

 summits, and then letting her sink in the intermediate abyss. 

 Observe a sea-fowl floating on the water, and the same effect will 

 be witnessed. If the water itself partook of the motion of the 

 waves, the ship and the fowl would each be carried forward as if 

 by a current, and would have the same progressive motion as the 

 liquid. Once on the crest of a wave, there they would constantly 

 remain, and their motion would be as smooth as if they were pro- 

 pelled upon the calm surface of a lake ; or if once in the hollow 

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