ch a full area of solid stone would stand. If this
pressure should be enormously increased after excluding the water, it
would finally result in crushing the stone into a solid mass; and if the
pressure should be increased indefinitely, some theoretical point would
be reached, as above noted, where the stone would eventually be
liquefied and would assume liquid properties.

[Illustration: FIG. 10.]

[Illustration: FIG. 11.]

The same general reasoning applies to pure sand, sand being in effect
cobblestones in miniature. In pressing the piston down on dry sand it
will be displaced into every existing abnormal void, but will be
displaced into these voids rather than pressed into them, in the true
definition of the word, and while it would flow out of an orifice in the
sides or bottom, allowing the piston to be forced down as in a
sand-jack, it would not flow out of an orifice in the top of the piston,
except under pressures so abnormally high as to make the mass
theoretically aqueous. If the positions of cylinder and piston be
reversed, the piston pointing vertically upward and the sand "bled" into
an orifice in or through it, the void caused by the outflow of this sand
would be filled by sand displaced by the piston pressing upward rather
than by sand from above.

It was the knowledge of this principle which enabled the contractors to
jack up successfully the roof of a long section of the cast-iron lined
tubes under Joralemon Street in Brooklyn, in connection with the
reconstruction of the Battery tubes at that point, the method of
operation, as partly shown in Fig. 2, Plate XXVIII, being to cut through
a section of the roof, 4 by 10 ft. in area, through which holes were
drilled and through which again the sand was "bled," heavy pressure
being applied from below through the medium of hydraulic jacks. By a
careful manipulation of both these operations, sections of the roof of
the above dimensions were eventually raised the required height of 30
in. and permanently braced there in a single shift.

If water in excess be put into a cylinder containing sand, and pressure
be applied thereto, the water, if allowed to flow out of an orifice,
will carry with it a certain quantity of sand, according to the
velocity, and the observation of this might easily give rise to the
erroneous impression that the sand, as well as the water, was flowing
out under pressure, and, as heretofore stated, has caused many engineers
and contrac