pressure, for instance, would be equal at every point, while the
pressure on the stone would be through and along the lines of contact.
If this contact was reasonably well made and covered 40% of the area,
one would expect the stone, independently of the water, to stand 40% of
the pressure which 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