friction of the sand on the piston. After several trials of this,
however, it was clearly seen that the friction was too slight to be
noted accurately by a gauge registering single pounds, that is, with a
piston in contact with 6 in. of sand vertically, a friction of 25 lb.
per sq. ft. would only require an increase of 1.8 lb. on the gauge. It
is therefore assumed that the friction on so small a piston in sand need
not be considered as a material factor in the experiments made.

The piston was plunged into clear water, and it was found that the
pressure required to lift it was about 4 lb. The cap was then taken off,
a depth of about 2 in. of sand was placed in the bottom of the chamber,
and then the piston was set in place and surrounded by sand to a depth
of some 6 in., water being added so that the sand was completely
saturated. This was allowed to stand until it had regained the stability
of ordinary sand in place, whereupon the cap with the collar bearing was
set in place over the piston, the machine was coupled up, and the pump
was started. A series of four experiments, extending over a period of
two or three days, gave the following results:

_Test 1._--The piston began to move at a pressure of 25 lb. The pressure
gradually dropped to 7½ lb., at which point, apparently, it came out of
the sand, and continued at 7½ lb. during the remainder of the test.

_Test 2._--The piston was plunged back into the sand, without removing
the cap, and allowed to stand for about 2 hours. No attempt was made to
pack the sand or to see its condition around the piston, it being
presumed, however, that it had reasonable time to get a fair amount of
set. At slightly above 20 lb. the piston began to move, and as soon as a
pocket of water accumulated behind the piston the pressure immediately
dropped to 9 lb. and continued at this point until it came out of the