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 81/2 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
scale in order to measure accurately the pressures due to sands and
earths of varying characteristics, it appears to be conclusive in
showing the principle, and near enough to the theoretical value to be
taken for practical purposes in designing structures against water
pressures when buried in sand or earth.

It should be carefully noted that the friction of the water through
sand, which is always a large factor in subaqueous construction, is
virtually eliminated here, as the water pressure has to be transmitted
only some 6 or 8 in. to actuate the base of the piston, whereas in a
tunnel only half submerged this distance might be as many feet, and
would be a considerable factor.

It should be noted also that although the area subject to pressure is
diminished, the pressure on the area remaining corresponds to the full
hydrostatic head, as would be shown by the pressure on an air gauge
required to hold back the water, except, of course, as it may be
diminished more or less by friction.

The writer understands that experiments of a similar nature and with
similar apparatus have been tried on clays and peats with results
considerably higher; that is, in one case, there was a pressure of 40
lb. before the piston started to move.

The following is given, in part, as an analysis and explanation of the
above experiments and notes:

It is well known that if lead be placed in a hy