228 HYDROGRAPHIOAL SURVEYING [chap. ix. 



low-water spring mark. If we reduce our soundings 2 feet 

 below this to the 1-foot mark, we shall be pretty certain not to 

 give too much water on shoal spots. 



An approximation of this kind would be, of course, noted 



on the chart when sent home, and also the manner in which 



the rise of spring-tides, which would be given as 14 feet, has 



been obtained. 



Rougher In still rougher work, an approximation of the rise of the 



mat^o^n of ^^'^^ may be got by having a marked boat-hook held upright 



Datum, at the water-line at time of low water ; the observer then places 



his eye at the high-water mark on the beach, and reads the 



mark on the boat-hook, where the horizon line cuts the latter, 



which will be the fall of the tide that day below high-water 



mark. If it is the high- water mark of the day that is so used, 



the result is the range of the tide for the day ; and if the distance 



that the spring's mark is above the day high-tide mark can be 



measured, we can arrive at the full rise and fall, as in the last 



article. 



This may be very useful in making a hurried plan of a 



bay, and thus the height of the water can be got by the officer 



putting in the coast-line from time to time during the day, 



without delaying liim much, and to the great advantage of the 



correctness of the soundings being taken at the time. 



Estima- The " Vulgar establishment " is an exceedingly loose term, 



eTw' h ^^ given on the charts. As it is strictly only on days 



ment. when the moon's mer. pass, is 12'\ or 0^\, that it can be directly 



observed, the surveyor is obliged to approximate to it in 



most cases. Tliis perhaps matters the less from the fact that 



the establishment, even when correctly obtained, is seldom 



invariable. 



The best way to approximate is to project the line of lunitidal 

 intervals, and measure the length of the abscissae from XIP. 

 and 0^\ for the vulgar establishment, meaning them if we get 

 more than one. 



If the tides are regular, especially as regards the semi- 

 menstrual inequality, the establisliment may be roughly 

 determined by a method given in the article in the Tide Tables, 

 from an observation of the tide at any period of the moon's 

 transit, but which we shall not further discuss, as, in a case 

 where it would be required, we should not know whether the 



