[Illustration:  PLATE XXVII, FIG. 1.—­EXPERIMENT SHOWING PROPERTIES OF SAND.]

[Illustration:  PLATE XXVII, FIG. 2.—­SAND PUSHED UP FROM BOWL BY WATER PRESSURE THROUGH GOOSE-NECK.]

If, then, the covering of sand over a structure is sufficiently heavy to allow arching action to be set up, the structure against which the pressure is applied must be relieved of much of the pressure of water against the area of sand not constituted as voids acting outside of the arching area.  This is confirmed by the two following experiments: 

Experiment No. 5.—­The same apparatus was used here as in Experiment No. 2, Fig. 7, except that the inside bucket had a solid bottom.  The inside and outside buckets were filled with water and the point was noted at which the weight would balance the inside bucket at a point some 3 in. off the bottom of the outside bucket.  This point was measured, and the bottom of the larger bucket was covered over with sand so that in setting solidly in the sand the inside bucket would occupy the same relative position as it did in the water.  The same weight was then applied and would not begin to lift the inner bucket.  For instance, in the first part of the experiment the weight stood at 12 in. from the pivot, while in the next step the weight, standing at the end of the bar, had no effect, and considerable external pressure had to be exerted before the bucket could be lifted.  Immediately after it was relieved, however, the weight at 12 in. would hold it clear of the sand.  No attempt was made to work the bucket into the sand; the sand was leveled up and the bucket was seated on it, turned once or twice to insure contact, and then allowed to stand for some time before making the experiment.  No attempt was made to establish the relationship between sands of varying voids, the general fact only being established, by a sufficient number of experiments, that the weight required to lift the bucket was more than double in sand having 40% of voids than that required to lift the bucket in water only.

[Illustration:  FIG. 9.]

Experiment No. 6.—­The apparatus for this experiment consisted essentially of a hydraulic chamber about 8 in. in diameter and 1 ft. high, the top being removable and containing a collar with suitable packing, through which a 21/2-in. piston moved freely up and down, the whole being similar to the cylinder and piston of a large hydraulic jack, as shown in Fig. 1, Plate XXVIII.  Just below the collar and above the chamber there was a 1/2-in. inlet leading to a copper pipe and thence to a high-pressure pump.  Attached to this there was a gauge to show the pressure obtained in the chamber, all as shown in Fig. 9.  The purpose of the apparatus was to test the difference in pressure on any object 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.