THE IMPENETRABLE SEA 



one on each side of the earth, with the result that any 

 part of the world's coastline affected by tides has two 

 tides a day. As the moon's phases take a lunar month to 

 go through, the large spring tides occur fortnightly. 



Sweeping round the world, the tides caused by the sun 

 and moon in this strange triangular association with the 

 earth, are influenced by land configurations and many 

 other factors. In confined channels the rushing currents 

 of water may be terrifyingly powerful. Where any strong 

 current races over rough, shallow ground, or where two 

 currents meet, the fierce and noisy condition known as 

 "tide-rip" may be created, or actual whirlpools may 

 occur, menacing shipping and human life. 



So many factors control the movements of tidal waters 

 that calculations designed to predict their future be- 

 haviour need to be so complex that they might seem, at 

 first thought, to be mathematically impossible. Such 

 calculations can only be compared with those made by 

 astronomers in forecasting the movements of stars, 

 planets and comets, yet oceanographers in the world's 

 meteorological observatories and institutes — particularly 

 at Liverpool and Birkenhead — have used tidal informa- 

 tion collected in past centuries with such good effect that 

 their calculations enable them to predict with accuracy 

 how tides will run in various parts of the world for many 

 years ahead. Unexpected happenings may disturb some 

 of their calculations and produce tidal waves causing 

 considerable damage, but the fact remains that ships of 

 all nations are able to time their movements by reference 

 to tide-tables which are- a monumental tribute to the 

 patience and intellectual skill of meteorological experts. 



The waves which break rhythmically on seaside 

 beaches in calm weather are subject to laws which have 

 been studied and analysed by man, even as the huge 

 tidal waves which swoop down upon coastal places at 

 rare intervals and cause great damage and loss of life are 

 also subject to known laws, although the movements of 



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