456 HYDROGRAPHIOAL SURVEYING [chap, xx 



tical difficulties there are likely to be with the gauge. Similar 

 gauges have been used with perfect success by Endeavour 

 and Hearty, using as much as 400 feet of tubing between sea 

 and gauge.* No difificulties were experienced in keeping 

 the vacuum, even when, as a severe test, it was left without 

 attendance for a whole month. In practice, however, it is 

 advisable to thoroughly wash through the whole apparatus 

 once a week or oftener if there is much sediment in the 

 water, as it is surprising what an amount of small particles 

 of shell and other things find their way in. The washing through 

 is easily performed by causing the water to syphon. 



Fig. 109 shows the tide-gauge as set up at Uyea Sound, 

 Shetland Islands. 



On an upright placed above high-water was secured a casing 

 to contain the gauge (door omitted in sketch). 



A is the glass barometer tube, B the column of mercury, the 

 height of which can be read off on the scale, C. 



D is a four- way connecting piece, one end of which is con- 

 nected to the glass barometer tube, one to the rubber-tubing, 

 E, which latter leads down the beach to the water, one is blank 

 flanged as not required, and the other is connected to F, the 

 copper cyhnder, which acts as an air -trap, on the front of which 

 is seen a glass gauge for telling the height of the water inside, 

 and underneath it is a tap. 



TIDAL OBSERVATIONS FROM A SHIP AT ANCHOR. 



The question of tidal observation in deep water having 

 recently engaged attention at the Hydrographic Department 

 of the Admiralty, an apparatus has been devised based on the 

 principle of the pneumatic tide-gauge referred to at p. 223. This 

 apparatus, for use from a ship at anchor, consists of indiarnbber 

 tubing having a bore of about | inch, supplied in a sufficient 

 number of lengths joined together to allow one end open to the 

 sea to be attached to a weight lowered to the bottom near the 

 anchor. The inboard end of the tubing is attached to the 

 upper part of a closed vertical cyhnder, 4 inches in diameter, 



* On one occasion when in a gale of wind the tide-pole was knocked 

 down, the gauge remained working satisfactorily. 



