and washers had been tightened down, the box was lifted off the floor.
There was found to be no tendency whatever for the bottom to fall away,
showing conclusively that the arching properties had not been destroyed
by the saturation of the sand.

The next three experiments were intended to show the relative pressure
over any given area in contact with the water in the one case or sand
and water in the other.

[Illustration: FIG. 8.]

_Experiment No. 4._--The apparatus for this experiment consisted of a
3-in. pipe about 4-in. long and connected with a ¾-in. goose-neck pipe
17 in. high above the top of the bowl shown in Fig. 8 and in Fig. 2,
Plate XXVII. A loose rubber valve was intended to be seated on the upper
face of the machined edge of the bowl and weighted down sufficiently to
balance it against a head of water corresponding to the 17-in. head in
the goose-neck. The bowl was then to be filled with sand and the
difference, if any, noted between the weight required to hold the
flap-valve down under the same head of water flowing through the sand.
The results of this experiment were not conclusive, owing to the
difficulty of making contact over the whole area of the sand and the rim
of the bowl at the same time. At times, for instance, less than 1 lb.
would hold back the water indefinitely, while, again, 2 or 3 lb. would
be required as opposed to the 4½ lb. approximate pressure required to
hold down the clear water. Again, at times the water would not flow
through the neck at all, even after several hours, and after increasing
the head by attaching a longer rubber tube thereto. In view of these
conditions, this experiment would not be noted here, except that it
unexpectedly developed one interesting fact. In order to insure against