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XXI. On the compressibility of Water. By Jacob Perkins, Esq „ 
Communicated by the late Right Hon. Sir Joseph Banks, Bart. 
G. C. B. P. R. S. 
Read June 29, 1&20. 
H a v i n g believed for many years that water was an elas- 
tic fluid, I was induced to make some experiments to as- 
certain the fact. This was done by constructing an instru- 
ment which I call a piezometer, and which is represented in 
Plate XXXII, Fig. 1. The cylinder. A, was three inches di- 
ameter, and eighteen inches long. The end, B, was made 
water tight by means of a plate which was soldered firmly to 
it. At the other end, C, a cap was made to screw on and off at 
pleasure ; being also made water tight. The rod or plunger, 
D, which was five-sixteenths of an inch in diameter, was made 
to pass through a tight stuffing box, E. On the rod immedi- 
ately above the stuffing box, was fixed a flexible ring, a. A 
cannon, Fig. 2, of a sufficient size to contain the piezometer, 
was fixed vertically in the earth, the muzzle being left about 
eighteen inches above ground, and the touch-hole plugged 
tight. At the mouth a strong cap, A, was firmly screwed 
on. In the centre of this cap a small forcing pump, B, was 
tightly screwed, the piston of which was five-eighths of an 
inch in diameter. There was an aperture, C, in the cap, to 
introduce a valve for the purpose of ascertaining the degree 
of pressure. One pound pressure on this valve indicated 
