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Sect. 1V,1 



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TIDES. 



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An up 



used to make the observations more accurate: 



rlrrht tube, open below and above, may be placed in the 



water, reaching above the high water, and below the luw 



water (or two tubes 



one for high water and one for lo 



water, if this mode be more convenient) . In this tube iv 



..4. 



( 





carry an upright rod, or else must have attached to it a 

 string which passes upwards over a pulley and is stretched 

 by a wcnght ; and the part of the rod or of the siring which 

 is outside the tube must carry an index, which shall mark 

 on a vertical fixed scale the rise and fall of the float. 



By making the tube close below, except one or more 



small openings, the motion of the waves will very little 



surf 



or 



may be observed with much accuracy. 



4 . It may happen that the moment of the highest 

 lowest water is difficult to determine, either with or 

 without the tube, on account of the water, while near the 

 highest or lowest, stopping or hanging still, without 

 either rising or falling, or else rising and falling irregu- 

 larly. 



If there is a considerable time during which the water 



neither rises nor ftills decidedly, note the moment when it 

 ceases to rise, and the moment when it begins to fall,^ 

 and take the time half way between these for the time of 



high water. 



5. Another method is the followin 



■At 



tervals of time near the time of high water, for example, 

 every ten minutes, or every five minutes, let the height 



of high water be observed, say tor Halt an noui u 

 hour, and from the height so observed pick out 



the 



