It has been the practice lately, among some engineers in Boston, as well
as in New York City, to assume that water pressures on the underside of
inverts is exerted on one-half the area only. The writer, however, has
made it a practice first to lay a few inches of cracked stone on the
bottom of wet excavations in order to keep water from concrete which is
to be placed in the invert. In addition to the cracked stone under the
inverts, shallow trenches dug laterally across the excavation to insure
more perfect drainage, have been observed. Both these factors no doubt
assist the free course of water in exerting pressure on the finished
invert after the underdrains have been closed up on completion of the
work. The writer, therefore, awaits with interest the repetition of
Experiment No. 6, with water on the bottom of a piston buried in coarse
gravel or cracked stone.

As for the arching effect of sand, the writer believes that Mr. Meem has
demonstrated an important principle, on a small scale. It must be
regretted, however, that the box was not made larger, for, to the
writer, it appears unsafe to draw such sweeping conclusions from small
experiments. As small models of sailboats fail to develop completely
laws for the design and control of large racing yachts, so experiments
in small sand boxes may fail to demonstrate the laws governing actual
pressures on full-sized structures.

For some time the writer has been using a process of reasoning similar
to that of the author for assumptions of earth pressure on the roofs of
tunnel arches, except that the vertical forces assumed to hold up the
weight of the earth have been ascribed to cohesion and friction, along
what might be termed the sides of the "trench excavation."