82 



HYDROGRAPHY. 



[Sect. III. 



st?"dard should be noted in a very conspicuous and un-- 

 mistakcable manner as being so many feet below the 

 mean water level, and recorded as the low water standard 

 of the port. It is a quantity which would nearly corre- 

 spond with half the range of an ordinary spring tide, 

 a term often written without any direct reference to the 

 low water standard, and so ambiguous that it is to be 

 hoped it will soon disappear from the face of our charts. 

 With this standard, and the known daily height of the 

 tide above mean water level, soundings taken at anv 

 hour may be prepared for comparison with the depths 

 upon the chart by the simple formula 



E 



T r 



. ft 

 cosine I— , 



180 





Zir 



= the lot^^ water standard to which the chart is adapted. 

 = the height of tide for the day aboT8 mean water leyel. 

 = the duration of the tide. 

 — the time from high water previous. 



Or, enter the traverse table with the time from the 

 nearest high water as a course (allowing 5^ of arc to 



Where R 



r 



D 



t 



time) 



(half the range 



of tide for the day) as a distance ; in the latitude column 

 will stand a quantity which applied to the loio water 



of 



according as the arc is less 



or greaier than 90% will give the reduction required. 



V - 



If the arc exceeds 90*^ take its supplement. But it is to 

 be observed that all these corrections, although pre- 

 ferable to the old method of reducing soundings, are but 

 approximations. In many places, especially in such as 

 have great tides, it is nocessarv to distinimish betweea 



rising ^adfall 

 If in a conn' 



e 



arthquakes, carefully watci 



i 





) 



s 



^^^ 



lit] 



the ti^^'^ 



lOfl 



Be^^' 



A-* 



of a 



rivei 



a 



har 



> 



<f' 



tion 



of 



25. H 



■u^-- 



1 



rolling 

 and rusl 

 even to 



u 



^m- ^ 



FT-* 



.1-Jj 



height ♦ ■ 

 resi'ect 1 



.*. 



n 



below t] 



tides; : 

 correct.^ 



26, ( 



periodic 



occa="-. 



the 

 level 



r 



coiii 



Oj 



'I tile be 



^ 



