Just here the writer desires to protest against considering semi-aqueous
masses, such as soupy sands, soft concrete, etc., as exerting
hydrostatic pressure due to their weight in bulk, instead of to the
specific gravity of the basic liquid. For instance, resorting again to
the illustration of cubes and spheres, it may be assumed that a cubical
receptacle has been partly filled with small cubes of polished marble,
piled vertically in columns. When this receptacle is filled with liquid
around the piles of cubes there will be no pressure on the sides except
that due to the hydrostatic pressure of the water at 62½ lb. The bottom,
however, will resist a combined pressure due to the water and the weight
of the cubes. Again, assume that the receptacle is filled with small
spheres, such as marbles, and that water is then poured in. The pressure
due to the weight of the solids on the bottom is relieved by the loss in
weight of the marbles due to the water, and also to the tendency of the
marbles to arch over the bottom, and while the pressure on the sides is
increased by this amount of thrust, the aqueous pressure is still that
of a liquid at 62½ lb., and it is inconceivable that some engineers, in
calculating the thrust of aqueous masses, speak of it as a liquid
weighing, say, 120 or 150 lb. per cu. ft.; as well might they expect to
anchor spherical copper floats in front of a bulkhead and expect the
hydrostatic pressure against this bulkhead to be diminished because the
actual volume and weight of the water directly in front of the bulkhead
has been diminished. Those who have had experience in tying narrow deep
forms for concrete with small wires or bolts and quickly filling them
with liquid concrete, must realize that no such pressures are ever
developed as would correspond to liquids of 150 lb. per cu. ft. If the
solid material in any liquid is agitated, so that it is virtually in
suspension, it cannot add to the pressure, and if allowed to subside it
acts as a solid, independently of the water contained with it, although