a stoppage of water, as above referred to, gravel was first put into the
bottom of the bowl and the flap-valve was then rubbed down and held
tightly while the pipe was filled. On being released, the pressure of
water invariably forced out the whole body of sand, as shown in Fig. 2,
Plate XXVII. Care was taken to see that the sand was saturated in each
case, and the experiment was repeated numberless times, and invariably
with the same result. The sand contained about 40% of voids. The
deduction from this experiment is that the pressure of water is against
rather than through sand and that any excess of voids occurring adjacent
to a face against which there is pressure of water will be filled with
sand, excepting in so far, of course, as the normal existing voids allow
the pressure of the water to be transmitted through them.

[Illustration: PLATE XXVII, FIG. 1.--EXPERIMENT SHOWING PROPERTIES
OF SAND.]

[Illustration: PLATE XXVII, FIG. 2.--SAND PUSHED UP FROM BOWL BY
WATER PRESSURE THROUGH GOOSE-NECK.]

If, then, the covering of sand over a structure is sufficiently heavy to
allow arching action to be set up, the structure against which the
pressure is applied must be relieved of much of the pressure of water
against the area of sand not constituted as voids acting outside of the
arching area. This is confirmed by the two following experiments:

_Experiment No. 5._--The same apparatus was used here as in Experiment
No. 2, Fig. 7, except that the inside bucket had a solid bottom. The
inside and outside buckets were filled with water and the point was
noted at which the weight would balance the inside bucket at a point
some 3 in. off the bottom of the outside bucket. This point was