222 



HYDROGRAPHICAL SURVEYING [chap. ix. 



Position 

 for Tide- 

 Pole. 



Tide- 

 Poles. 



If the tides are regular, we shall find that on days on which 

 the moon passes the meridian at the same time, the times and 

 heights of high and low water will be the samo. 



This knowledge is very valuable in many surveys where 

 from local causes Ave cannot always have a tide-pole going, as 

 from previous observation we can, when the tides have been 

 found to be regular, construct a table founded on moon's 

 meridian passage, from which we can take out a reduction for 

 soundings when working on a small scale. 



When we arrive on our surveying ground, then, one of the 

 first things to do will be to set up a tide-pole, whatever is going 

 to be the character of our observations. 



For this we want a sheltered spot, if we can find one, and 

 also firm ground on which to place it, as nothing is more 

 annoying than to find the pole down, especially when out of 

 sight of the ship, when the tide-watchers, unassisted, generally 

 succeed in putting it up again in a different position. 



If a pier is available, there is nothing so simple and satis- 

 factory as a plank secured to it, marked in feet and inches, the 

 former being painted red, white, and blue alternately, with 

 bold black figures. 



If we have no pier, an ordinary spar, shod with an iron spike 

 and painted as above, driven as far into the ground as possible 

 and well stayed to heavy weights, anchors, rocks, or whatever 

 we can get, will stand well, and generally answers our practical 

 purposes. This may sometimes be so placed as to be read from 

 the ship with a glass. 



If, however, there is no shelter and much wash of the sea, 

 and accurate observations are required, we must use a tube 

 of some kind. 



A square one of deals can be knocked up on board ; but it 

 must not be too small, as we shall want a slit down one side, 

 through which an indicator fixed to a rod carried by a fioat 

 inside may work, and the water washing in by this slit will 

 destroy the value of the tube, unless the area of it be large 

 enough to make the water thus admitted too insignificant in 

 quantity to disturb practically the surface of the water inside. 

 Where there is not much range of tide, the slit can be dispensed 

 with, and the rise and fall marked by an indicator protruding 

 from the top of the tube (which in this case could be a boiler 



