connection therewith.

The experiment in which the plunger on the hydraulic ram is immersed in
sand and covered with water does not seem to be conclusive. By this
experiment the author attempts to demonstrate that the pressure of the
water transmitted through the sand is only about 40% as great as when
the sand is not there. The travel of ground-water through the earth is
at times very slow, and occasionally only at the rate of from 2 to 3 ft.
per hour. In the writer's opinion, Mr. Meem's experiment did not cover
sufficient time during which the pressure was maintained at any given
point. It is quite probable that it may take 15 or 20 min. for the full
pressure to be transmitted through the sand to the bottom of the
plunger, and it is hoped, therefore, that he will make further
experiments lasting long enough to demonstrate this point.

In regard to the question of skin friction on caissons and piles, it may
be of interest to mention an experiment which the writer made during the
sinking of the large caissons for the Williamsburg Bridge. These
caissons were about 70 ft. long and 50 ft. wide. The river bottom was
about 50 ft. below mean high water, and the caissons penetrated sand of
good quality to a depth of from 90 to 100 ft. below that level. On two
occasions calculations were made to determine the skin friction while
the caissons were being settled. With the cutting edge from 20 to 30 ft.
below the river bottom, the calculations showed that the skin friction
was between 500 and 600 lb. per sq. ft. The writer agrees with Mr. Meem
that, in the sinking of caissons, the arch action of sand is, in a great
measure, destroyed by the compressed air which escapes under the cutting
edge and percolates up through the material close to the sides of the
caissons.