Naturally, the writer doubts the author’s conclusions as to the pressure at the top of large square caisson shafts when he states that “the pressure at the top * * * will * * * increase proportionately to the depth.”  Again, the author is apparently not conversant with experiments made by the Dock Department of New York City, concerning piles driven in the Hudson River silt, which showed that a single heavily loaded pile carried downward with it other unloaded piles, driven considerable distances away, showing that it was not the pile which lacked in resistance, as much as the surrounding earth.

In conclusion, the writer heartily concurs with the statement that “too much has been taken for granted in connection with earth pressures and resistance,” and he is sorry to be forced to add that he believes the author to be open to the criticism which he himself suggests, that “both in experimenting and observing, the engineer [and in this case the author] will frequently find what is being looked for or expected and will fail to see the obvious alternative.”

FRANCIS L. PRUYN, M. AM.  SOC.  C. E. (by letter).—­Mr. Meem should be congratulated, both in regard to the highly interesting theories which he advances on the subject of sand pressures—­the pressures of subaqueous material—­and on his interesting experiments in 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.