220 



HYDROGRAPHICAL SURVEYING [chap. ix. 



Tide 

 Tables. 



Local 

 Circum- 

 stances. 



Observa- 

 tion Indis 

 pensable. 



Defini- 

 tions. 



thing, and those for obtaining additional data for scientific 

 investigation are another. 



We shall mainly concern ourselves with the former, where 

 much rougher observations are usually admissible ; but here, 

 again, it must depend upon the scale and nature of our chart 

 what degree of nicety is requisite. 



A few words on the Theory of the Tides will be given at 

 the end of the chapter. 



The reader is referred to Dr. Whewell's treatise on the 

 tides, published in the preliminary part of the Admiralty 

 Tide Tables, for much information respecting their move- 

 ment. 



A regular series of observations, even for our practical work, 

 should be taken if possible ; but in many cases the necessity 

 for leaving tide-watchers encamped is inconvenient, and may 

 be unhealthy, and we may have to be satisfied by obtaining 

 what will be sufficient to enable us to construct the chart, 

 which is our immediate business. 



In other cases we may only be staying a few days at a 

 place, as when making a plan of a small isolated harbour. 



What we absolutely require in making a chart, is to know 

 the height of the water whilst sounding is going on, above the 

 level of low-water springs, which is called the " datum for 

 reduction." 



We shall also wish to ascertain, if possible, the " establish- 

 ment," which is the time of high-water at full and new moon, 

 called in the charts " High- water at full and change "; the 

 rise of spring-tides above our datum ; and the range of the 

 tides at neaps, and the time occupied by the rise and the 

 fall of each tide, as these will give valuable information to 

 the navigator. 



We may here give definitions of some of the terms used in 

 speaking of the tides. 



" Rise " of a tide is the height of the high- water level 

 above the low spring datum. 



" Spring rise," as given on the Admiralty chart, implies 

 the height to which ordinary spring-tides rise above the datum 

 to which the soundings are reduced. This in all cases should 

 be L. W. 0. S. 



The height to which extraordinary spring-tides rise above 



