454 



HYDROGRAPHICAL SURVEYING [chap. xx. 



(a) Prevent leakage as much as possible by painting over all 

 connections with several coats of " shellac varnish," 

 made by breaking up sealing-wax into small pieces, 

 placing in a bottle, and covering with methylated 

 spirits, allowing to stand for two or three days before 

 use. 



_-_-//4;V trap 



Fig. 1 08. 



(6) Connect a closed copper cj^linder to the highest portion 

 of the system to act as an '" air trap," and fill it with 

 water. Any air that leaks into the system rises into 

 the cyUnder, displacing a certain amount of water, 

 but not affecting the reading of the gauge. A gauge- 

 glass fixed outside the cylinder shows the amount of 

 air and water inside. A screw plug is provided in the 

 top of the cylinder for filling purposes, and a tap under- 

 neath the cyhnder. 



This gauge is not intended to supersede the ordinary tide- 

 pole, but only to provide an alternative means of readily 

 recording the tide w^hen, for instance at night, it is inconvenient 

 or impossible to obtain the direct readings of the pole. It is 

 most useful, of course, on a shelving beach, as the mercury 

 gauge, etc., can be fixed up inside the tide watcher's hut or 

 tent, or as convenient, at some distance from the water, the 

 limit being the amount of tubing available. The less height, 

 however, the mercury gauge is above high-water, the less prac- 



