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, 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.