e
used to cover the contingencies of vibration, oscillation, etc., to
which such structures may be subjected and which may ultimately allow
leads of water to work their way underneath.

On the other hand, he urges that, in cases of floor areas of deeply
submerged structures, such as tunnels or cellars, the pressure to be
resisted should be assumed to be only slightly in excess of that
corresponding to the pressure due to the water through the voids.

The question of pressure, etc., in Class B, or semi-aqueous materials
will be considered next. Of these materials, as already shown, there are
two types: (_a_) sand in which the so-called quicksand is largely in
excess of any normal voids, and (_b_) plastic and viscous materials. The
writer believes that these materials should be treated as mixtures of
solid and watery particles, in the first of which the quicksand, or
aqueous portion, being virtually in suspension, may be treated as water,
and it must be concluded that the action here will be similar to that of
sand and pure water, giving a larger value to the properties of water
than actually exists. If, for instance, it should be found that such a
mixture contained 40% of pure water, the writer would estimate its
pressure on or against a structure as (_a_) that of a moist sand
standing at a steep angle of repose, and (_b_) that of clear water, an
allowance of 60% of the total volume being assumed, and the sum of these
two results giving the total pressure. Until more definite data can be
obtained by experiments on a larger scale, this assumed value of 60% of
the total volume for the aqueous portion may be taken for all conditions
of semi-aqueous materials, except, of course, where the solid and
aqueous particles may be clearly defined, the pressures being computed
as described in the preceding pages.

As to the question of pure quicksand (if such there be) and other
aqueous materials of Class C, such as water, oil, mercury, etc., it has
already been shown that they are to be considered as liquids of their
normal specific gravity; that is, in calculating the air pressure
necessary to displace them, one should consider their specific gravity
only, as a factor, and not the total weight per volume including any
impurities which they might contain undissolved.

In order to have a clearer conception of aqueous and semi-aqueous
materials and their action, they must be viewed under conditions not
ordinarily apparent. For ins