224 HYDROGRAPHICAL SURVEYING [chap. ix. 



figure (Fig. 60). The pressure of air being sufficient to expel the 

 water from the lower end of the flexible tubing at the maximum 

 height to which the tide rises, a continuous stream of air slowly 

 escapes into the water whilst the apparatus is in action. 



Bubbles of air will then be observed rising to the surface of 

 the water in the Klinger's water-gauge glass, indicating thereby 

 that the air is escaping freely through the tubing. 



The bight of a short length of tubing between the Klinger's 

 water-gauge and the closed vessel containing the mercury is 

 carried well above the level of the water-gauge, in order to pre- 

 vent the possibility of water being carried up if the air-pressure 

 should be turned on suddenly, or with too much force. 



The iron float in the vertical pipe projecting from the 

 mercury vessel is of considerable weight, and rises or falls in 

 accordance with the air-pressure that is acting at the moment 

 on the surface of the mercury. 



The movement of the iron float is recorded by a pencil on a 

 cylinder revolved by a clock. The pencil is secured to a sub- 

 stantial metal block working freely in guides. 



The air escapes through a hole about iV inch in diameter in 

 a metal plug screwed into the end of the flexible tubing. 



This metal plug is secured to a sinker in such a manner as to 

 keep it off the bottom, and clear of mud or weeds which might 

 choke the orifice, and is dropped in a depth of from 3 to 6 feet 

 at low-water. 



It has been found by experiment that the daily loss of air- 

 pressure from the reservoir is rather large at first, but steadily 

 diminishes. The loss is always roughly one-tenth of the total 

 pressure remaining day by day. Thus, starting with 200-pound 

 pressure, the first day it loses 20 pounds ; by the time the 

 pressure gets down to 100 pounds it will be losing 10 pounds a 

 day, and at 50 pounds only 5 pounds a day. When the reservoir 

 gets below 50 pounds' pressure, it should be recharged, as it is 

 found that with pressures as low as 30 or 40 pounds the 

 readings were always too small near high- water. This minimum 

 probably varies with the actual depth at high-water over the 

 sinker ; in the case referred to this depth was 27 feet. 



The scale of height on the recording cylinder is obtained from 

 actual experiment, and when once determined it should be 

 constant. 



