If the water is capable of exerting its full hydrostatic pressure
through the sand, the total pressure would be the full hydrostatic
pressure on the bottom of the piston where in contact, and, where
separated from it by a grain of sand, the pressure would be decreased
only by the weight of the grain. If a large proportion of the top area
of a grain is in contact, as assumed by the author, this reduction of
pressure would be very small. A correct interpretation can be obtained
only after more complete experiments have been made.

For horizontal pressures exerted by saturated sands on vertical walls,
it has not been demonstrated that anything should be deducted from full
water pressure. No matter how much of the area is in direct contact with
the sand rather than the water, the full water pressure would be
transmitted through each sand grain from its other side and, if
necessary, from and through many other grains which may be in turn in
contact with it. The pressure on such a wall will be water pressure over
its entire surface, and, in addition, the thrust of the sand after
correcting for its loss of weight in the water.

The fact that small cavities may be excavated from the sides of trenches
or tunnels back of the sheeting proves only that there is a local
temporary arching of the material, or that the cohesion of the particles
is sufficient to withstand the stress temporarily, or that there is a
combination of cohesion and arching. The possibility of making such
excavations does not prove that pressure does not exist at such points.
That sand or earth will arch under certain conditions has long been an
accepted fact. The sand arches experimented with developed their
strength only after considerable yielding and, therefore, give no index
of the distribution or intensity of stress before such yielding.