he
stresses due to the thrust of the material will" not "change if bracing
should be substituted for the material in the area" designated by him,
etc., provided he makes the further assumption that absolutely no
motion, however infinitesimal, has taken place meantime; but, unless
such motion has actually taken place, no arch action can have developed.
An arch thrust can result only with true arch action, that is, with
stable abutments, and the mass stressed wholly in compression, with
corresponding shortening of the arch line. The arch thrust must be
proportional to the elastic deformation (shortening) of the arch line.
If any such arch as is shown in Fig. 5 is assumed to carry the whole of
the weight of material above it, that assumed arch must relieve all the
assumed arches below. Therefore each of the assumed arches can carry
nothing more than its own mass. Otherwise the resulting thrust would
increase with the depth, which is opposed to the author's theory.

Turning again to the condition that each arch can carry only its own
weight: if these arches are assumed of thicknesses proportional to the
distance upward from the bottom of the wall, they will be similar
figures, and it is easily demonstrated that the thrust will then be
uniform in amount throughout the whole height of the wall, except,
perhaps, at the very top. This condition is contrary to the author's
ideas and also to the facts as demonstrated by the writer's experiment
on the 40-ft. retaining wall at St. George. Consequently, the author's
statement: "nor can anyone * * * doubt that the top timbers are stressed
more heavily than those at the bottom," is emphatically doubted and
earnestly denied by the writer. Furthermore, "the assumption" made by
the author as to "the tendency of the material to slide" so as to cause
it "to wedge * * * between the face of the sheeting * * * and some plane
between the sheeting and the plane of repose," is considered as
absolutely unwarranted, and consequently the whole conclusion is
believed to be unjustified. Nor is the author's assumption (line 5, p.
361), that "the thrust * * * is measured by its weight divided by the
tangent of the * * * angle of repose" at all obvious.

The author presents some very interesting photographs showing the
natural surface slopes of various materials; but it is interesting to
note that he describes these slopes as having been produced by the
"continual slipping down of particles." The vast