CHAP. XX.] TIDAL OBSERVATIONS 457 



and about 6 feet high, on the top of which is fitted a small 

 Boiu'don gauge of ordinary pattern. The lower part of the 

 vertical cyh'nder is in connection with an air-reservoir, and 

 is also connected, by a separate pipe of small diameter, with a 

 large Bourdon gauge of special construction. 



The air-reservoir, charged by a powerful air-pump, consists 

 of four cylinders, each of which is similar in size and pattern 

 to the vertical cylinder. The large Bourdon gauge is 12 inches 

 in diameter, very deUcately made, capable of indicating pres- 

 sures up to 250 pounds on the square inch, and graduated on 

 a reflecting surface to obviate the effect of parallax in reading 

 off. It can be accurately read to within iV pound. The 

 method of using the apparatus is as foUows : 



With the shij) lying at anchor, and having sufficient cable 

 veered, the indiarubber tubing should bear no strain. The 

 12-inch Bourdon gauge being shut off by a needle valve, con- 

 trolling connection with the remainder of the apparatus, air 

 is pumped into the air-reservoir flowing from thence to the sea 

 through the vertical cyUnder and indiarubber tubing. Pump- 

 ing is continued until the small Bourdon gauge ceases to rise,' 

 thereby showing that all the water is expelled from the tubing, 

 and that the air is escaping freely from the submerged end 

 at each stroke of the pump. 



The valve at the junction of the indiarubber tubing and 

 vertical cyh'nder, controlling connection with the sea, is then 

 closed, and the an* reservoir and vertical cylinder charged to a 

 pressm'e considerably exceeding that of the head of water due 

 to the depth. 



The compressed air being then admitted to the 12-inch 

 Bourdon gauge by turning the needle valve, the whole apparatus 

 is again placed in direct communication with the sea by means 

 of the valve for that purpose. 



The air pressure as shown by the 12-inch Bourdon gauge wiU 

 then steadily faU as the air escapes into the sea and will con- 

 tinue to do so until the pressure in the apparatus exactly 

 balances that due to the column of water represented by the 

 depth over the submerged end of the indiarubber tubing. 

 When the pointer of the 12 -inch gauge ceases to fall and remains 

 quite stationary the gauge is read off. 



As a column of sea-water 1 foot high, with sectional area of 



