276 THE TIDES. 



Taking Courtown in Wicklow as the standard — a spot remarkable as the 

 node or axis of the great tidal wave of the St. George's Channel, and where 

 there is little or no rise or fall ; at Ballycastle on the North, the mean sea 

 level is higher by 0-881 foot, and lower on the South at Castle Townsend 

 by 0-938 foot, than it is at Courtown. Thus the mean level is nearly 1 foot 

 10 inches higher on the North than it is on the South of Ireland. Of 

 course this fact has no bearing upon the seaman's application of tidal 

 phenomena, but it is curious. 



(232.) It has been found that a low barometer causes a higher Tide and 

 the reverse. This element in the disturbance of the regular Tides, the 

 effects of atmospheric pressure, has been estimated by different observers, 

 and its amount has been ascertained with considerable accuracy. Thus, 

 at Liverpool, there is a difference in the height oi high water of 10-1 inches 

 for a variation of 0-91 in the barometer ; and at London it has been calcu- 

 lated by Mr. Dessiou that the water rises 6-3 inches for -90 depression of 

 the barometer. M. Dausssy has ascertained that, at Brest, the ocean rises 

 -223 metre, or 8-78 inches, for a depression of 0-158 metre, or -622 inch, in 

 the barometer.* These results are nearly identical with those ascertained 

 by Sir James Eoss in the Arctic regions, in 1848, by means of the steady 

 level of the winter ice. These refinements in tidal calculations are, perhaps, 

 of little practical value for the mariner when at sea ; they may be useful 

 in entering a dock ; but they are of the utmost service in generalizing the 

 phenomena of the Tides, upon which so little, it may be said, is known 

 that may be applied. 



(233.) The foregoing are the principal effects of the causes which produce 

 the Tides in reference to their rise and fall. There is another branch of 

 the subject, however, which is of great importance to the navigator ; that 

 is, the currents formed by the alternate elevation and depression of the 

 ocean. As before mentioned, in the open sea it may be considered that 

 there is no tidal current, and that the tidal wave is propagated without 

 any actual displacement in the particles of the water. But when this wave 

 approaches the coast, the case is widely different, and the wave must 

 necessarily form a current, sometimes flowing in one direction, and at 

 others in the opposite one. This variation in the progress of the flood and 

 ebb tide-wave must vary with every locality, and is influenced by the par- 

 ticular configuration of the coasts, &c., by which it passes. The question 

 of the form and transmission of waves is so complicated, and involving 

 mathematical analysis of so high an order, that it cannot be usefully dwelt 

 on here. 



Upon the direction in which the great tidal wave is propagated, we at 

 present have much to learn. It was supposed by the late Sir J. Lubbock, 

 that it travels from the Cape of Good Hope to Gibraltar in twelve hours ; 

 from Gibraltar to Edinburgh in about twelve hours ; and from Edinburgh 

 to London in about twelve hours,! which is in accordance with Bernouilli's 

 theory. Passing North-Eastward from the South Atlantic, it strikes the 

 S.W. shores of Great Britain and Ireland, and becomes divided by these 



• Phil. Trans., 1836, pp. 220, 221 ; and Conn, des Temps, 1834. 

 t Phil. Trans., 1836, page 218. 



