On Water and Air. 
523 
1880.J 
that cloud in this apparatus; although it is composed of 
transparent carbonic acid, still it will appear almost dark 
for the time being. To produce this effeCt we must lower 
the pressure very suddenly ; and observe, the instant I do so 
the space above the liquid is filled with a dense cloud. 
And now I have to refer to an instrument which we 
employ in this experiment. It is what we call the hydraulic 
press. Its construction you will readily understand from 
this diagram (Fig. 43). A strong iron cylinder, c, with very 
thick sides, has inserted into it an iron ram, P, working 
water-tight in the collar of the cylinder. On the ram, P, 
Fig. 43. 
there is an iron table, b, on which the substance to be 
pressed is placed. Strong iron columns support an iron 
plate, A, fixed direftly over the table, b. By means of a 
small pipe, T, the cylinder, c, which is filled with water, 
communicates with a small force-pump, P , which is worked 
by means of the lever, H. When the lever H is lifted, the 
piston p' ascends, the valve v opens, and the cylinder 01 the 
pump is filled with water. When the lever is forced down 
ao-ain, the valve v closes, and the water is forced through a 
valve’ in the pipe T into the cylinder c, and the piston P, 
carrying the table b, is forced upwards. By a repetition of 
this adtion enormous pressure can be obtained. When it is 
necessary to relieve the pressure, the screw plug inserted 
into the tube t' is opened, and the water flows from the 
cylinder c, through the tube t', into the cistern, and the 
