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2. On a Method of Levelling. 
A gauge, similar to a steam-gauge, 1s attached to a fine 
india-rubber pipe 22 yds. long. At the other end is a glass 
tube a few inches long with a fixed mark upon it, and just 
below the glass tube the pipe expands into a small india- 
rubber bulb. The observer holds the gauge and advances 
22 yds., his assistant remains behind and holds the other ex- 
tremity of the tube, the tube being filled with water. The 
gauge indicates the pressure due to the difference of Level. 
By squeezing the bulb the water can always be brought up to 
the fixed mark in the glass tube G. The observer and his 
assistant always hold the gauge and the glass tube, say at the 
height of the eye in each observation. Each lh. of pressure 
corresponds to a difference of Level of nearly 2ft. The number 
of feet corresponding to the pressures are marked upon the 
gauge, from 0 up to + 60ft. on the right hand and down to 
60 ft. on the left hand. In the figure the gauge points to 
+ 20, indicating that the observer is 20 ft. below the position 
of his assistant; had it been — 20, he would have been 20 ft. 
above him. » 
