218 THE TIDES. 



Horn, and round the whole shore of Terra-del-Fuego, from the western ex- 

 tremity of Magellan's strait to Staten Island, it is very remarkable that the 

 tidal wave, instead of following the moon in its diurnal course, travels to the 

 eastward. This, however, is a partial phenomenon ; and a little farther to the 

 north of the last-named places, the tides set to the north and west. In the 

 Mediterranean and Baltic seas the tides are inconsiderable, but exhibit irregu- 

 larities for which it is difficult to account. The Indian ocean appears to have 

 high water on all sides at once, though not in the central parts at the same 

 time. 



Since the tide* on our coast are derived from the oscillations produced under 

 the direct agency of the sun and moon in the Southern ocean, and require a 

 certain interval of time for their transfer, it follows that, in general, the tide is 

 not due to the moon's transit immediately preceding, but is regulated by the 

 position which the sun and the moon had when they determined the primary 

 tide. The time elapsed between the original formation of the tide and its ap- 

 pearance at any place is called the age of the tide, and sometimes, after Ber- 

 noulli, the retard. On the shores of Spain and North America, the tide is a 

 day and a half old ; in the port of London, it appears to be two days and a half 

 old when it arrives. 



VELOCITY OF THE TIDE WAVES. 



In the open ocean the crest of tide travels with enormous velocity. If the 

 whole surface were uniformly covered with water, the summit of the tide wave, 

 being mainly governed by the moon, would everywhere follow the moon's 

 transit at the same interval of time, and consequently travel round the earth 

 in a little more than twenty-four hours. But the circumference of the earth 

 at the equator being about 25,000 miles, the velocity of propagation would 

 therefore be about 1,000 miles per hour. The actual velocity is, perhaps, no- 

 where equal to this and is very different at different places. In latitude 60 

 south, where there is no interruption from land (excepting the narrow promonto- 

 ry of Patagonia), the tide wave will complete a revolution in a lunar day, and 

 consequently travel at the rate of 670 miles an hour. On examining Mr. 

 Whewell's map of cotidal 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 ap- 

 pears to be 10 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.) It is scarce- 

 ly necessary to remind the reader that the above velocities refer to the trans- 

 mission of the undulation, and are entirely different from the velocity of the 

 current to which the tide gives rise in shallow water. 



RANGE OF THE TIDE. 



The difference of level between high and low water is affected by various ( 



causes, but chiefly by the configuration of the land, and is very different at dif- ( 



rent places. In deep inbends of the shore, open in the direction of the tide < 



'J 



