142 OUR HERITAGE THE SEA 



great celestial bodies, either in opposition or conjunc- 

 tion, causes the waters of the various oceans to advance 

 upon or recede from the various shores of the whole 

 world with such regularity that the navigator can 

 calculate with certainty the time of high water at any 

 given port with a very small amount of trouble and 

 mathematical knowledge. 



But water being so mobile an element, innumerable 

 complexities occur, due to local peculiarities, to strong 

 and persistent winds, etc., and it is the consideration of 

 these peculiarities that makes the study of the tides so 

 interesting. Moreover, in many parts of the world 

 current and tide meet and act upon one another, intro- 

 ducing further complications, and rendering the sea- 

 man's task of allowing for the mysterious movements of 

 the great body of water upon which he floats by no means 

 an easy one. Yet one more complication, which arises 

 from the meeting of the incoming tide from the ocean 

 and the down-rushing torrent from a river. It will 

 sometimes happen that, owing to an extraordinarily 

 heavy rainfall, a river will be so full of water as to 

 rush with much more than its usual impetuosity sea- 

 ward, and, meeting the ascending flood of salt water, 

 will struggle to beat it back. That being impossible, 

 a compromise is effected by the level of the water 

 rising much higher than usual, overflowing its banks, 

 and spreading devastation all around. The same 

 result may be brought about by a gale of wind blowing 

 directly up a river and aiding the incoming tide by 

 pushing it inland far beyond the usual tidal limits. 

 These limits vary, of course, with the amount of fall 

 or gradient a river has towards the sea ; but whatever 

 the limit may be, in a civilized country, at any rate, 



