CHAP. IX.] 



TIDES 



227 



time and height of high or low water to be arrived at much 

 nearer than by simple observation. 

 Thus, suppose we have noted — 



X. 



..XI. 



We project these as in accompanying Fig. 61, and by drawing 

 a horizontal line from the X. 10 position to the opposite side 

 of the curve, bisecting it, and letting fall a perpendicular to the 

 line of time, we find X. 32 as the time of high water. The 

 compass, measuring the highest point of the curve, gives a 

 little over 1 3 feet 1 inch as the height marked on the pole. 



Fig 6i. 



13— 



,0" 



-o- — o. 



p- 



io M 30 40 M Xl!o 



If we are at the place during the spring-tides, we can get a Approxi- 

 fair low-water datum by observation, and all soundings will Datum! * 

 be reduced to that, by the height marked on the pole above 

 this datum, at the time the soundings were taken each day, 

 being subtracted from them. But it may happen that we 

 arrive at the place a few days after a spring-tide, and leave 

 again before the next one. The only thing to do is to note the 

 high-water mark on the shore, and ascertain by measurement 

 how far it is above the high tide of the day as marked also on 

 the shore, subtract the same quantity from the low- water mark 

 on the pole of that day, and call that the low-water spring 

 datum, subtracting perhaps a foot or two extra to be on the 

 safe side. 



Thus, suppose at high water our pole marks 13 feet 1 inch, 

 and the high-water mark on the beach is 2 feet 6 inches above 

 the level of the sea at that time ; at low water the pole marks 

 5 feet 8 inches. This will give us 3 feet 2 inches as the probable 



15—2 



