CHAP. IX.] TIDES 223 



tube), and marking on a scale lashed so as to project above the 

 tube. The water would be admitted by holes bored near the 

 bottom of the tube if it is to be placed on muddy ground. 



The zero of the tide-pole must be frequently compared with 

 the fixed datum mark to detect any sinkage of the pole. If 

 stepped on a wooden framework, there is less liability of this 

 occurring on soft ground. 



Many attempts have been made to devise an automatic tide- Auto- 

 gauge suitable for use in the conditions ordinarily met with by ^^6° 

 surveying ships. Gauge, 



At present the most promising apparatus is a pneumatic 

 arrangement consisting, in its simplest form, of an india-rubber 

 tube, one end of which is secured to a weight, lowered to 

 the bottom in a depth of about a fathom at low-water, and the 

 other end connected to an air-pump placed in a tent on the 

 beach, a Bourdon pressure-gauge being introduced close to 

 the air-pump. Air is pumped in until it attains sufficient 

 pressure to escape by expelling the water in the tubing, 

 which will be indicated by the Bourdon pressure-gauge ceasing 

 to rise. 



A scale of pressure corresponding to different depths is formed 

 by noting the readings of the pressure-gauge as the tide rises on 

 the tide-pole. It may be more readily determined by lowering 

 the end of the tubing with the weight attached in deep water 

 to different depths, and noting the corresponding readings of 

 the pressure-gauge. 



The next development was to admit compressed air from a 

 reservoir by turning a cock when required to make an observa- 

 tion, instead of using a pump. Subsequently a continuous 

 stream of compressed air was allowed to escape slowly through 

 the tubing, the supply being dra^^'n from a reservoir of three 

 cylinders connected together, and charged by a powerful hand- 

 pump to a pressure of 100 to 150 pounds per square inch, thus 

 giving a continuous indication of the height of the tide by 

 reading the pressure-gauge. The three cylinders contained 

 sufficient air to last from ten days to a fortnight. 



The instrument in its final stage is now under trial, an 

 attempt having been made to replace the Bourdon gauge by 

 something more sensitive and efficient. This has been attained 

 by means of a mercury recorder, as shown in the accompanying 



