CHAP. XX.] VACUUM TIDE GAUGE 453 



it was found that the railway iron struck the bottom, owing 

 to the motion of the Hghter. 



A somewhat similar method of sweeping was made use of at 

 Gibraltar, using a 70-feet railway iron slung fore and aft under 

 a lighter, the lighter being moved broadside on by ropes, thus 

 obtaining the advantage of maximum length of sweep. 



VACUUM TIDE GAUGE. 



By EEAE-ADMIKAL H. E. PUREY-CUST, C.B., Late 

 Htdrographer of the Navy. 



To one end of a length of rubber-tubing attach a glass baro- 

 meter tube, place the latter vertically in a vessel of water above 

 high-water mark, and lead the tubing down the beach to the sea. 

 With very little difficulty the water may be made to syphon 

 from the vessel to the sea, provided the highest portion of the 

 tubing is not more than about 30 feet above the level of 

 the sea. 



Whilst the water is syphoning pour some mercury into the 

 vessel (a jam-pot makes a convenient vessel), and then raise 

 the latter until the end of the barometer tube dips under the 

 mercury ; the mercury will at once rise in the tube to a height 

 AB (Fig. 108), equivalent to the height of a corresponding 

 column of water CD. If CD varies owing to the surface of 

 the water at D varying, then AB must hkewise vary in the 

 above proportion. Consequently, if D represent the surface of 

 the tide, as the tide rises and falls so will the column of mercury 

 AB fall and rise in the proportion of about one inch of mercury 

 to one foot of water, and therefore, if a suitable scale be pro- 

 vided, AB may read off hke a tide -pole. 



The end of the tubing must always be below the surface of 

 the water, but it is immaterial how much ; it should, however, 

 be kept clear of the bottom if of mud or sand, and may with 

 advantage be attached to the under surface of a small buoy 

 moored in a convenient spot. 



As a very small amount of air leaking into the tubing will 

 largely affect the vacuum, and, consequently, the height of the 

 mercury column, two precautions are necessary, viz. — 



