, he will be convinced
that many of the assumptions which he cannot at present endorse are
based on fact, and his co-operation will be welcomed with the greatest
interest. Among the experiments which he is asked to make is the one in
dry sand, noted as Experiment No. 3, whereby it can be shown very
conclusively that additional back-fill will result in increased arching
stability, on an arch which would collapse under lighter loading.

The writer is indebted to Mr. Goodrich for pointing out some errors in
omission and in typography (now corrected), and for his hearty
concurrence in some of the assumptions which the writer believed would
meet with greatest disapproval.

In reply to Mr. Pruyn and Mr. Gregory, the writer assumed that the
piston area in Experiment No. 6 should be reduced only by the actual
contact of material with it. If this material in contact should be
composed of theoretical spheres, resulting in a contact with points
only, then the theoretical area reduced should be in proportion to this
amount only. The writer does not believe, however, that this condition
exists in practice, but thinks that the area is reduced very much more
than by the actual theoretical contact of the material. He sees no
reason, as far as he has gone, to doubt the accuracy of the deductions
from this experiment.

Regarding the question of the length of time required to raise the
piston, he does not believe that the position of his critics is entirely
correct in this matter; that is, it must either be conceded that the
piston area is cut off from the source of pressure, or that it is in
contact with it through more or less minute channels of water. If it is
cut off, then the writer's contention is proved without the need of the
experiment, and it is therefore conclusive that a submerged tunnel is
not under aqueous pressure or the buoyant action of water. If, on the
other hand, the water is in contact through channels bearing directly
upon the piston and leading to the clear water chamber, any increase in
pressure in the water chamber must necessarily result in a virtually
instantaneous increase of the pressure against the piston, and therefore
the action on the latter should follow almost immediately. In all cases
during the experiments the piston did not respond until the pressure was
approximately twice as great as required in clear water, therefore the
writer must conclude either that the experiments proved it conclusively
or