CHAP. IX.] TIDES 249 



tion at one spot may enable predictions for that spot to be 

 made, the variations are sometimes so great that at a short 

 distance the phenomena are entirely different. 



All prediction may be upset by what is known as the 

 meteorological tide — that is, the variation in the height caused 

 by winds, and by the difference in pressure of the air on the 

 surface of the water. 



The inconsistencies in the tide thus caused affect the height 

 of the water more than the time of high or low water, and as 

 they affect the mean level of the water, an unusually high tide 

 induced by them does not mean an unusually low tide, but the 

 reverse. 



Wind will naturally have more effect when there is a funnel- 



shaped estuary than when it blows on to a straight, open coast, 



the heaped-up water at the wide mouth being forced higher and 



higher as it advances, for under such circumstances water will 



run up-hill. 



From the foregoing it is evident that the mean level of the 5'^ean 



Level, 

 water will considerably vary. 



When steady winds blow at certain times of the year, the 

 variation in mean level will be seasonal ; in other places it may 

 be constantly varying with the direction of the wind. 



Tliis variation in the mean level is important as regards 

 navigation in some places. For instance, when a shallow flat 

 exists which must be crossed to gain access to a harbour, and 

 which at ordinary high water affords just sufficient depth, a 

 change in the mean level may cause the high-water level to 

 be 1, 2, or even 3 feet less than usual. 



It is, therefore, most necessary for surveyors to acquaint 

 themselves with the effect of wind at such places, and to 

 record it. 



Round the coasts of England and Scotland mean sea-level is 

 usually a few inches above Ordnance datum, the Ordnance . 

 datum being 4-67 feet above the level of the old dock-sill at 

 Liverpool. In some places, such as Sheerness and Grimsby, 

 the difference is as much as 21 inches. 



In cases where the rise of tide is great, in a funnel-shaped "* 

 estuary much encumbered by sandbanks, and where there is a 

 continuous outflow caused by a large river, the phenomena of 

 the " bore " appears. 



