sand.

_Test 3._--The piston was plunged into the sand and hammered down
without waiting for the sand to come to a definite set. In this case the
initial pressure shown by the gauge was 17½ lb., which immediately
dropped to 8 lb. as soon as the piston had moved sufficiently far to
allow water to accumulate below it.

_Test 4._--The cap was again removed, the piston set up in place, the
sand compacted around it in approximately the same condition it would
have had if the sand had been in place underground; the cap was then set
in place and, after an hour, the pump was started. The pressure
registered was 25 lb. and extended over a period of several seconds
before there was any movement in the piston. The piston responded
finally without any increase of pressure, and, after lifting an inch or
two, the pressure gradually dropped to 10 lb., where it remained until
the piston came out of the sand.

The sum and average of these tests shows a relation of 22 lb. for the
piston in sand to about 8½ lb. as soon as the volume of water had
accumulated below it, which would correspond very closely to a sand
containing 40% of voids, which was the characteristic of the sand used
in this experiment.

The conclusions from this experiment appear to be absolutely final in
illustrating the pressure due to water on a tunnel buried in sand,
either on the arch above or on the sides or bottom, as well as the
buoyant effect upon the tunnel bottom under the same conditions.

While the apparatus would have to be designed and built on a much larger