148 OUR HERITAGE THE SEA 



an island in the way or by some prominent headland 

 breaking and deflecting the full force of the tidal 

 wave, will be found to account for the immunity of 

 all our more prosperous harbours on the west coast 

 from abnormal variations in the height of their water- 

 level; for such variations are fatal to regular trade, 

 being so often disastrous in their effects upon shipping. 

 On the east coast, of course, no such tidal troubles 

 annoy. The whole mass of the British Isles inter- 

 venes to prevent the tidal wave rushing in straight 

 from its ten thousand mile sweep. It comes, of 

 course, as it must do everywhere (almost), but it 

 comes gently, regularly, and unless, as sometimes 

 happens in the estuary of the Thames, for instance, 

 it is aided by an easterly gale, it never plays any 

 pranks upon the shipping in the river or the house- 

 holders along the banks. But it quietly raises the 

 water-level and enables the largest ships to get up 

 to their docks, whose gates are opened punctually to 

 tide time and closed again before the water has begun 

 to fall, so that ships of the largest tonnage, safe behind 

 those massive barriers, may lie afloat and discharge 

 their cargoes. This utilization of the tides by man 

 for the docking of ships has been of very great 

 influence in the history of navigation ; for, while 

 the smaller vessels may lie aground without any 

 serious harm to their fabric, it always means much 

 inconvenience in the handling of their cargoes ; and, 

 as the incidence of high water necessarily varies in 

 time each day, there is the annoyance of the tide 

 serving for handling the cargo in the middle of the 

 night while the day has been wasted. But the larger 

 vessels cannot be treated thus casually, and, moreover, 



