x The Hydrosphere 161 



hour. Admiralty charts show the tidal data for each sea- 

 port, thus, e.g., " High Water, Full and Change, X. rise 10 

 feet." This means that on the day of Full Moon and of 

 Change or New Moon high water occurs at 10 A.M., and the 

 rise of the sea between low water and high water is 10 

 feet. Each successive high tide after Full Moon occurs at 

 an interval of about 12^ hours, rises to a somewhat less 

 height and falls to a somewhat less depth, thus covering 

 and laying bare a narrower strip of the beach until the 

 Moon's phase is the third quarter, when the time of morn- 

 ing high water is 4 A.M. and neap-tide occurs. After this 

 the tides increase in amplitude again until the period of 

 Change or New Moon, when the time of morning high water 

 is once more 10 o'clock. The time during which tidal 

 currents run in one direction and in the opposite bears 

 little relation to the hours of high water and low water, 

 depending largely on the form of the coast. In partially 

 enclosed seas a branch of the tidal wave usually enters by 

 each channel, as shown in the co-tidal map of the British 

 Islands (Plate XVII.) 



219. Tides in Bays and Estuaries. When the tidal 

 wave of the ocean enters a narrowing bay or sea inlet, the 

 depth of which diminishes rapidly, the tidal currents become 

 rapid and tumultuous and the water is heaped up to a great 

 depth against the land. At the entrance of the Bay of Fundy 

 the tide rises 8 or 9 feet, but at the head the rise at spring- 

 tides is more than 70 feet, the greatest tidal range known. 

 The highest spring-tide at Cardiff docks rises 42 feet, and 

 the lowest neap-tides 20 feet, while at the mouth of the 

 Bristol Channel the rise of spring-tide is only about 10 

 feet. The tidal wave rushes up some rivers with great 

 violence, forming a bore or wall of foaming water stretching 

 right across the stream, and often producing much destruc- 

 tion to shipping in the Amazon and Yang-tse-kiang. A 

 tidal current sweeping through a narrow irregular channel 

 gives rise to eddies or whirlpools sometimes of great size, 

 like that of the Maelstrom in the Lofoten Island group. 



220. Properties of Water. In order to understand 

 the action of solar energy on the hydrosphere, we must 



M 



