226 



HYDROGRAPHICAL SURVEYING [ciiAr. ix. 



Fixed 

 Mark for 

 Reference 



Time of 

 Observa- 

 tions. 



High and 

 Low 

 Water 

 Observa- 

 tions. 



Graphic 

 Method. 



By cutting out the automatic recording arrangement and 

 using in its place a Bourdon gauge graduated to 300 pounds' 

 pressure, the apparatus has been used on the ship at anchor 

 in 35 fathoms, the rise and fall of tide being accurately 

 determined by the difference of pressure recorded on the gauge. 

 Half-hourly observations were made during several days ; the 

 results when plotted on squared paper practically agreed with 

 thi' curve shoAvn by observations on a tide-pole on shore. 



Whenever it can be done, a mark should be made on some 

 fixed object near the tide-pole, corresponding to some mark 

 on the pole, which can then be replaced in the same position 

 if it accidentally gets displaced. 



Levels should also be carried to some permanent mark in the 

 vicinity, and the difference of level between this mark and the 

 datum given in the chart, with the object of enabling future 

 surveys to be reduced to the same datum level. This is most 

 important. When, as is the case of many civilised countries, 

 there is a fixed plane of reference for land surveys, and bench 

 marks are available, the tidal datum should always be con- 

 nected with such fixed plane. 



The level of the water on the tide-gauge should be noted 

 every hour, if we are going to make a regular series, both night 

 and day. If simply to get a datum for soundings, day observa- 

 tions only are necessary, except at springs, when it is as well to 

 get the high and lovv- water at night also, as night tides in some 

 places and at some seasons are lower or higher than the day 

 ones. The tide-watch should be set to mean time of place. 



It is not amiss in any case, when nothing is known of the 

 tides, to observe for twenty-four hours, at half-hour intervals, 

 as a commencement, as this will tell us whether the tides are 

 regular or not, and we can take observations accordingly. 



To get the time and height of high and low water accurately, 

 observe every ten minutes, for half an hour or so, before and 

 after high and low water, and calculate from these records the 

 exact time and height required. 



This is best done by projecting graphically thus : Divide a 

 line into equal parts to represent hours and minutes, and from 

 this, at the corresponding time, set off at right angles distances, 

 on any chosen scale, to represent the height of tide registered 

 at that time. These spots, joined by a curve, will enable the 



