, after a cave-in, always wait until the
ground has settled and compacted itself before tunneling, usually with
apparent safety, over the scene of the cave-in.

The writer quotes as follows from Mr. Goodrich's discussion:

"In any case, no arch action can be brought into play until a
certain amount of settlement has taken place so as to bring the
particles into closer contact, and in such a way that the internal
stresses are practically those only of compression, and the
shearing stresses are within the limits possible for the material
in question."

Further:

"Consequently, an almost infinitesimal settlement of the
'centering' may cause the complete destruction of an arch of
earth."

And further:

"On the other hand, it is believed that the author's statement, as
to the 'tendency of marbles to arch,' * * * should be qualified by
the addition of the words, 'only when a certain amount of
deflection has taken place so as to bring the arch into action.'"

In a large measure the writer agrees with the first and last quotations,
but sees no reason to endorse the second, as it is impossible to
consider any arch being built which does not settle slightly, at least,
when the "centers" are struck.

Regarding his criticism of the lack of arching action in balls or
marbles, he seems to reason that the movement of the marbles would
destroy the arch action. It is very difficult for the writer to conceive
how it would be possible for balls or marbles to move when confined as
they would be confined if the earth were composed of them instead of its
present ingredients, and under the same conditions otherwise. Mr.
Goodrich can demonstrate the correctness of the writer's theories,
however, if he will repeat the writer's Experiment No. 3, with marbles,
with buckshot, and with dry sand. He is also advised to make the
experiment with sand and water, described by the writer, and is assured
that, if he will see that the washers are absolutely tight before
putting the water into the box, he can do this without bringing about
the collapse of the arch; the only essential condition is that the
bottom shall be keyed up tightly, so as not to allow the escape of any
sand. He is also referred to the two photographs, Plate XXIV,
illustrating the writer's first experiment, showing how increases in the
loading resulted in compacting the material of the arch and in the
consequent