AN AUTOMATIC TIDAL INDICATOR 



This purely American aid to navigation automatically shows the height of the water at 

 any moment. When the arrows point upward the tide is rising; when downward, it is falling. 

 The index on the scale shows that at the moment when the photograph was taken there was 

 an excess of 1^2 feet above the normal level of the water. 



the process is about as follows : A party- 

 goes out in a rowboat or launch, among 

 its members being two observers with 

 sextants and a map showing the shore- 

 line and the objects whose positions have 

 been determined by triangulation ; a re- 

 corder with a clock and record book ; a 

 leadsman and a steersman. The officer 

 in charge directs the recorder to make 

 a note of the position of the boat, which 

 is determined by the observers, and the 

 leadsman casts his line and calls out the 

 depth in feet or fathoms as he draws it 

 up. The recorder makes a note of this 

 and also of the course along which the 

 boat is headed. At intervals of a minute 

 or more the leadsman casts his lead, 

 while every three or four minutes the 

 observers take observations until the end 



of the course is reached, where a final 

 set of observations locate the end of the 

 line. The boat then turns and runs 

 other lines in the same way, until the 

 entire bottom of the surveyed area has 

 been sounded (see page 657). 



The resulting figures must be cor- 

 rected so that they will all apply to what 

 is known as "mean low water." This is 

 fixed by taking the sum of the low-water 

 readings for perhaps thirty days and 

 finding their average. In fixing the 

 depth of a sounding, the time the sound- 

 ing was taken is noted and deductions 

 or additions are made to conform with 

 the state of the tide at that hour, as 

 shown on the tide gauge sheets, so that 

 all soundings are recorded on the chart 

 as if made at "mean low water." On 



659 



