CHAPTER IX 



TIDES 



All soundings in published charts are given for low-water Tidal 

 at ordinary spring-tides ; we therefore want all the infor- tjon^g^om- 

 mation we can get about the tides, and the very first thing mencsd at 

 to be done on arriving on the surveying ground is to com- 

 mence observations on them. 



There are very few parts of the world in which we have 

 absolutely no knowledge of the tidal movement, so we have 

 generally something to commence upon — that is, we usually 

 know within an hour or two the time of high-water at full 

 and new moon, called H. W. F. and C. on the charts, as 

 except in estuaries or peculiarly shaped coasts, this will not 

 differ greatly from places near at hand, and the same may be 

 said for the range of the tide. 



It will altogether depend upon our length of stay in any Different 

 locality as to what we can hope to find out about the tides. ^ionsTor 

 To get full information requires observation during months Different 

 in succession, as in many parts the tides vary considerably at j^^^^^' 

 different times of year. The number of high and low tides in 

 a day in certain places departs from the normal phase of 

 from six to seven hours for each rise or fall ; in others the 

 tide will take longer to rise or fall than vice versa, etc. A 

 long series of this kind is therefore very valuable, as the 

 tidal theories are at present far from fulfilling all the require- 

 ments of observation all over the world, and good data are 

 much wanted ; but it is not often that the surveyor can obtain 

 such a series. 



It will be seen, then, that tidal observations for the practical 

 reduction of soundings for purposes of navigation are one 



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