that
the silt exerted full hydrostatic pressure. At the same time, W.I. Aims,
M. Am. Soc. C. E., stated in a public lecture, and recently also to the
writer, that in 1890 he made some tests of the pressure of this silt in
normal air for the late W.R. Hutton, M. Am. Soc. C. E. A hole, 12 in.
square, was cut through the brickwork and the iron lining, just back of
the lock in the north tube (in normal air), and about 1000 ft. from the
New Jersey shore. It was found that the silt had become so firm that it
did not flow into the opening. Later, a 4-in. collar and piston were
built into the opening, and, during a period covering at least 3 months,
constant observations showed that no pressure came upon it; in fact, it
was stated that the piston was frequently worked back and forth to
induce pressure, but no response was obtained during all this period.
The conclusion must then be drawn that when construction, with its
attendant disturbance, has stopped, the solid material surrounding
structures tends to compact itself more or less, and solidify, according
as it is more or less porous, forming in many instances what may be
virtually a compact arch shutting off a large percentage of the normal,
and some percentage even of the aqueous, pressure.

That the pressure of normally dry material cannot be measured through
small openings can be verified by any one who will examine such material
back of bracing showing evidences of heavy pressure. The investigator
will find that, if this material is free from water pressure, paper
stuffed lightly into small openings will hold back indefinitely material
which in large masses has frequently caused bracing to buckle and
sheeting planks to bend and break; and the writer reiterates that such
experiments should be made in trenches sheeted with horizontal sheeting
bearing against short vertical rangers and braces giving horizontal
sections absolutely detached and independent of each other. In no other
way can such experiments be of real value (and even then only when made
on a large scale) to determine conclusively the pressure of earth on
trenches.

As to the questions of the relative thrust of materials under various
angles of repose, and of the necessity of dividing by the tangent, etc.;
these, to the writer, seem to be merely the solution of problems in
simple graphics.

The writer believes that if Mr. Goodrich will make, even on a small
scale, some of the experiments noted by the writer