CH.\p. IX.] TIDES 235 



moon's declination and moon's parallax, should be sought 

 for on the diagram. 



The lunitidal interval and range of tide corresponding to 

 the preceding transit of the moon for the day on that section 

 of the diagram where the above-mentioned conditions are 

 most nearly satisfied will give the approximate H. W. F. and C, 

 and range of tide for that day. The reduction to low water 

 for each hour of the tide can then be deduced by the method 

 described by Captain T. H. Tizard, R.N., C.B., F.R.S., on 

 p. 149 of the Admiralty Tide Tables. 



If the diagram does not include the particular combination 

 of conditions required, the predicted results may be consider- 

 ably in error ; but it is sometimes possible to apply rough 

 corrections for the differences between the astronomical con- 

 ditions for the day and those of the diagram by studying the 

 effect on the lunitidal interval and range of tide produced by 

 changes in the astronomical elements during the various luna- 

 tions shown on the diagram, if there are a sufficient number 

 to enable conclusions to be drawn. 



As a river is ascended, the range of tide tends to first increase Range of 



to a maximum, and then to decrease, until the undulation dis- 'E]^ ^*, 



Different 



appears entirely. Tide-poles for the reduction of soundings Points in 

 should therefore be erected at suitable intervals, and compared ^Tidal 

 with each other, and a note should be made of the spot where 

 the rise and fall ceases. 



Comparisons should be effected by noting the high and low Compari- 

 water« on all the tide-poles on the same days ; the more observa- ^°^ °^ 

 tions that can be obtained, the better. The results will show Poles, 

 the level of the river both at high and low water, especially if 

 time admits of a series of levels being taken from the lower to 

 the upper pole. 



When the range of tide is very large, it is often convenient Double 

 to use two or more tide-poles, placed near each other, but at ™^' 

 different levels, so that when one tide-pole is nearly covered 

 the other will show only 1 or 2 feet. Both tide-poles should be 

 read and recorded in the interval, during which they may be 

 compared. 



The indications of a tide-pole placed in river estuaries cannot Tide-Pole 

 be relied on to show the rise and fall of tide at any considerable i^ River 

 distance on either side along the coast. 



