542 Mr. Perkins on the progressive compression of 
descent of the piston occasions it to be water-tight, without 
need of any stuffing. The lever G, which in this view passes 
behind the pump, is part of the apparatus for indicating the 
force of compression. Fig 3, represents a section of the 
indicator. A, is a cylindrical tube, communicating with the 
receiver of the compressor, and of such dimensions that its 
area is equal to - 1 - of a square inch. Hence the number of 
pounds pressing on its piston by means of the lever G, indi- 
cates directly the number of atmospheres used for compres- 
sion ; since the arms of the lever are in proportion of 10 to 1, 
and its weight counterpoised by means of the hook O and 
weight, fig. 2, every pound in the scale represents 10 atmo- 
spheres of compressing force. 
For the purpose of measuring the diminution of bulk that 
water undergoes, a glass piezometer, fig. 4, is used for con- 
taining it. This consists of an elongated bulb, nearly four 
inches in length and about ^ of an inch in diameter, with a 
tube of regular bore, 9 inches long and about i of an inch 
diameter internally. By weighing the quantity of quicksilver 
contained in this instrument when full, and ascertaining the 
weight contained in a given length of the tube alone, it was. 
found that the whole content was equal to a tube of 190 in- 
ches long, having the diameter of the contracted part of the 
piezometer. Fig. 5, shows a section of the bottom of the 
piezometer, containing a small disk of steel D, and above it 
a delicate hair spring C, of sufficient strength to retain its 
position after being pressed upwards in the tube, so as to 
serve as register of the degree of compression that has been 
effected. Fig. 11, is a phial containing a small quantity of 
quicksilver, in which the piezometer, full of water, with its 
