TIDES AND OCEANIC HIGHWAYS. 



355 



parts of the mainland, where the shores are low. This is particularly observable along 

 the south coast of Lincolnshire and in the Bay of Lancaster. The latter estuary becomes 



Lancaster Sands. 



at low water an expanse of sand, across which there is a road, the channel of the river 

 Lune presenting the chief obstacle to the passage dry-shod. In other narrow seas, how- 

 ever, there is scarcely any perceptible tide at all, owing to the conditions under which 

 they communicate with the general sweep of the deep. The distribution and configuration 

 of the land, together with the influence of the winds, greatly involve the problem of the 

 tides, and render it one of the most difficult in the whole range of physics. We shall 

 merely mention a few of its leading features. 



The diurnal rotation of the earth being from west to east, the apparent course of the 

 moon is from east to west, and consequently in an ocean of considerable extent in that 

 direction, a tidal wave is formed following the lunar course. The only great belt of water 

 which answers to this condition is the Pacific Ocean, for the general direction of the 

 Atlantic is from north to south, its breadth from east to west being comparatively small. 

 It is the southern part of the Pacific, including the Indian Ocean, that exhibits the 

 greatest extent of surface in the direction of the moon's path ; and accordingly a very 

 regular tide-wave is there produced, the general course of which is from east to west, 

 but running towards the tropics, the region of the direct line of the lunar attraction. 

 From the mouth of the Red Sea to the Cape of Good Hope the whole east coast of Africa 

 is reached about the same time by the summit of a single tide-wave, causing the hours of 

 high water at its different stations to be coincident. It is otherwise with the tides of the 

 Atlantic, along the coasts of which the hours of high water are successively later as we 

 travel northward, that being the general direction pursued by its tidal waves, which the 



annexed diagram will explain. Sup- 

 pose AB a belt of water, and CD another 

 narrower belt opening into it, the 

 other lines being ridges of waves pass- 

 ing along AB. It is obvious that 

 when a wave has arrived at c, part of it 

 will run along the belt c D to find its 



level, and there will be a succession of waves in c D numerically the same as in A B, 

 but pursuing a different direction. The relation between the Atlantic and the Pacific 



if 



