"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.

Reference is again made to the diagram, Fig. 12, which shows that, in
most cases of coffer-dams in combined aqueous and earth pressure, there
may be nearly equal, and in some cases even greater, loading toward the
bottom.

The writer also specifically states that in air the difference between
aqueous and earth pressure is plainly noted by the fact that bracing is
needed so frequently to hold back the earth while the air is keeping out
the water.