14 THE SEA SHORE 



bodies must increase as their distance from the earth diminishes, 

 it follows that there must be occasional appreciable variations 

 in the vigour of the tidal movements of the ocean. 



As the earth rotates on its axis, while at the same time the 

 tidal wave must necessarily keep its position under the moon, this 

 wave appears to sweep round the earth with considerable velocity. 

 The differences in the level of the ocean thus produced would 

 hardly be appreciable if the earth were entirely covered with 

 water ; but, owing to the very irregular distribution of the land, 

 the movements of the tidal wave become exceedingly complex ; 

 and, when it breaks an entrance into a gradually narrowing 

 channel, the water is compressed laterally, and correspondingly 

 increased in height. It is thus that we find a much greater 

 difference- between the levels of high and low tides in continental 

 seas than are to be observed on the shores of oceanic islands. 



We have occupied so much of our time and space in ex- 

 planation of the movements of the tides not only because we 

 think it desirable that all who delight in sea-side rambles should 

 understand something of the varied motions which help to give 

 such a charm to the sea, but also because, as we shall observe later, 

 these motions are a matter of great importance to those who are 

 interested in the observation and study of marine life. And, seeing 

 that we are writing more particularly for the young naturalists 

 of our own island, we must devote a little space to the study 

 of the movements of the tidal wave round Great Britain, in order 

 that we may understand the great diversity in the time of high 

 tide on any one day on different parts of the coast, and see how 

 the time of high tide for one part may be calculated from that 

 of any other locality. 



Were it not for the inertia of the ocean and the resistance 

 offered by the irregular continents, high tide would always exist 

 exactly under the moon, and we should have high water at any 

 place just at the time when the moon is in the south and crossing 

 the meridian of that place. But while the inertia of the water 

 tends to make all tides late, the irregular distribution of the land 

 breaks up the tidal wave into so many wave-crests and greatly 

 retards their progress. 



Thus, the tidal wave entering the Atlantic round the Cape of 

 Good Hope mingles with another wave that flows round Cape 

 Horn, and the combined wave travels northward at the rate of 

 several hundred miles an hour. On reaching the British Isles 



