[Illustration: FIG. 1. SECTIONS OF BOX-FRAME FOR SAND-ARCH
EXPERIMENT]

_Experiment No. 1._--As the sand-box experiments described in the former
paper were on a small scale, exception might be taken to them, and
therefore the writer has made this experiment on a scale sufficiently
large to be much more conclusive. As shown in Fig. 1, wooden abutments,
3 ft. wide, 3 ft. apart, and about 1 ft. high, were built and filled
solidly with sand. Wooden walls, 3 ft. apart and 4 ft. high, were then
built crossing the abutments, and solidly cleated and braced frames were
placed across their ends about 2 ft. back of each abutment. A false
bottom, made to slide freely up and down between the abutments, and
projecting slightly beyond the walls on each side, was then blocked up
snugly to the bottom edges of the sides, thus obtaining a box 3 by 4 by
7 ft., the last dimension not being important. Bolts, 44 in. long, with
long threads, were run up through the false bottom and through 6 by 15
by 2-in. pine washers to nuts on the top. The box was filled with
ordinary coarse sand from the trench, the sand being compacted as
thoroughly as possible. The ends were tightened down on the washers,
which in turn bore on the compacted sand. The blocking was then knocked
out from under the false bottom, and the following was noted:

As soon as the blocking was removed the bottom settled nearly 2 in., as
noted in Fig. 1, Plate XXIV, due to the initial compacting of the sand
under the arching stresses. A measurement was taken from the bottom of
the washers to the top of the false bottom, and it was noted as 41 in.
(Fig. 1). After some three or four hours, as the arch had not been
broken, it was decided to test it under greater loading, and four men
were placed on it, four others standing on the haunches, as shown in
Fig. 2, Plate XXIV. Under this additional loading of about 600 lb. the