14 GENEEAL NAVIGATION. 



On coasts where there is much diurnal inequality in the tides the 

 amount of rise and fall can not be depended upon, and additional 

 caution is necessary. 



Mean sea level. — The important fact should be remembered that 

 the depths at half tide are practically the same for all tides, whether 

 neaps or springs. Half tide, therefore, corresponds with mean sea 

 level. This makes a very exact plane of reference, easily found, to 

 which it would be well to refer all high and low waters. 



The tide tables give, in Table 3. for all the ports, the plane of 

 reference to which tidal heights are referred and its distance below 

 mean sea level. 



If called on to take special soundings for the chart at a place where 

 there is no tidal bench mark, mean sea level should be found and the 

 plane for reductions established at the proper distance below it, as 

 ascertained by the tide tables, or by observations, or in some cases, 

 if the time be short, by estimation, the data used being made a part 

 of the record. 



Tidal streams. — In navigating coasts Avhere the tidal range is 

 considerable especial caution is necessary. It should be remembered 

 that there are indrafts to all bays and bights, although the general 

 run of the stream may be parallel with the shore. 



The turn of the tidal stream offshore is seldom coincident with the 

 time of high and low water on the shore. In some channels the tidal 

 stream maj'^ overrun the turn of the vertical movement of the tide by 

 ?) hovn-s, forming what is usually known as tide and lialf tide, the 

 effect of which is that at high and low water by the shore the stream 

 is running at its gi-eatest velocity. 



The effect of the tidal wave in causing currents may be illustrated 

 by two simple cases : 



(1) Where there is a small tidal basin connected with the sea by 

 a large opening. 



(2) Where there is a large tidal basin connected with the sea by a 

 small opening. 



In the first case the velocity of the current in the opening will have 

 its maximum value when the height of the tide within is changing 

 most rapidly, i. e., at a time about midway between high and low 

 water. The water in the basin keeps at approximately the same level 

 as the water outside. The flood stream corresponds with the rising, 

 and the ebb with the falling of the tide. 



In the second case the velocity of the current in the opening will 

 have its maximum value when it is high water or low water without, 

 for then there is the greatest head of water for producing motion. 

 The flood stream begins about three hours after low water, and the 

 ebb stream about three hours after high water, slack water thus 

 occurring about midway between the tides. 



