that his assumption is proved without the necessity of the
experiments. That is, the pressure is virtually not in evidence until
the piston has commenced to move.

Mr. Pruyn has added valuable information in his presentation of data
obtained from specific tests of the bearing value of, and friction on,
hollow steel piles. These data largely corroborate tests and
observations by the writer, and are commended to general attention.

Mr. Carter's information is also of special interest to the writer, as
much of it is in the line of confirming his views. Mr. Carter does not
yet accept the theory of increased pressure toward the top, but if he
will examine or experiment with heavy bracing in deep trenches in clear
sand, or material with well-defined angles of repose, he will probably
find much to help him toward the acceptance of this view.

The writer regrets that he has not now the means or appliances for
further experiments with the piston chamber, but he does not believe
that reliable results could be obtained in broken stone with so small a
piston, as it is possible that the point of one stone only might be in
contact with the piston. This would naturally leave the base exposed
almost wholly to a clear water area. He does not believe, however, that
in practice the laying of broken stone under inverts will materially
change the ultimate pressure unless its cross-section represents a large
area.

Mr. Perry will find the following on page 369:

"It should be noted also that although the area subject to pressure
is diminished, the pressure on the area remaining corresponds to
the full hydrostatic head, as would be shown by the pressure on an
air gauge."

This, of course, depends on the porosity of the material and the
friction the water meets in passing through it.

As to Mr. Thomson's discussion, the writer notes with regret two points:
(_a_) that specific data are not given in many of the interesting cases
of failures of certain structures or bracing; and (_b_), that he has
not in all cases a clear understanding of the paper. For instance, the
writer has not advocated the omission of bottom bracing or sheeting. He
has seen many instances where it has been, or could have been, safely
omitted, but he desires to make it clear that he does not under any
circumstances advocate its omission in good work; but only that, in
well-designed bracing, its strength may be decreased as it approaches
the bottom