submerged in clear water and on the same object buried in the sand under
water. It is readily seen that, if pressure be applied to the water in
this chamber, the amount of pressure (as measured by the gauge)
necessary to lift the piston will be that due to the weight of the
piston, less its displacement, plus the friction of the piston in the
collar.

[Illustration: PLATE XXVIII, FIG. 1.--APPARATUS FOR MEASURING LOSS
OF PRESSURE IN SUBAQUEOUS MATERIALS.]

[Illustration: PLATE XXVIII, FIG. 2.--RAISING ROOF OF BATTERY TUBES,
IN BROOKLYN, BY "BLEEDING" SAND THROUGH DISPLACED PLATES.]

Now, if for any reason the bottom area of the piston against which the
water pressure acts be reduced, it will necessarily require a
proportionate amount of increase in the pressure to lift this piston.
If, therefore, it is found that 10 lb., for illustration, be required to
lift the piston when plunged in clear water, and 20 lb. be required to
lift it when buried in sand, it can be assumed at once that the area of
the piston has been reduced 50% by being buried in the sand, eliminating
the question of the friction of the sand itself around the piston. In
order to determine what this friction might be, the writer arranged a
table standing on legs above the bottom of the chamber, allowing the
piston to move freely through a hole in its center. Through this table
pipes were entered (as shown in part of Fig. 9). The whole was then
placed in the chamber with the piston in place, and the area above was
filled with sand and water. It is thus seen that, the end of the piston
being free and in clear water, the difference, if any, between the
pressure required to lift the piston when in clear water alone and in
the case thus noted, where it was surrounded by sand, would measure the