The experiments noted below were made with varying material, though the principal type used was a fine sand, under the conditions in which it is ordinarily found in excavations, with less than 40% voids and less than 10% of very fine material.

[Illustration:  FIG. 7.]

Experiment No. 2.—­The first of these experiments, which in this series will be called No. 2, was simple, and was made in order to show that this material does not flow readily under ordinary conditions, when not coupled with the discharge of water under high velocity.  A bucket 12 in. in diameter, containing another bucket 9 in. in diameter, was used.  A 6 by 6-in. hole was cut in the bottom of the inner bucket.  About 3 in. of sand was first placed in the bottom of the larger bucket and it was partly filled with water.  The inside bucket was then given a false bottom and partly filled with wet sand, resting on the sand in the larger bucket.  Both were filled with water, and the weight, W, Fig. 7, on the arm was shifted until it balanced the weight of the inside bucket in the water, the distance of the weight, W, from the pivot being noted.  The false bottom was then removed and the inside bucket, resting on the sand in the larger one, was partly filled with sand and both were filled with water, the conditions at the point of weighing being exactly the same, except that the false bottom was removed, leaving the sand in contact through the 6 by 6-in. opening.  It is readily seen that, if the sand had possessed the aqueous properties sometimes attributed to sand under water, that in the inside bucket would have flowed out through the square hole in the bottom, allowing it to be lifted by any weight in excess of the actual weight of the bucket, less its buoyancy, as would be the case if it contained only water instead of sand and water.  It was found, however, that the weight, resting at a distance of more than nine-tenths of the original distance from the pivot, would not raise the inside bucket.  On lifting this inside bucket bodily, however, the water at once forced the sand out through the bottom, leaving a hole almost exactly the shape and size of the bottom orifice, as shown in Fig. 1, Plate XXVII.  It should be stated that, in each case, the sand was put in in small handfuls and thoroughly mixed with water, but not packed, and allowed to stand for some time before the experiments were tried, to insure the compactness of ordinary conditions.  It is seen from Fig. 1, Plate XXVII, that the sand was stable enough to allow the bucket to be put on its side for the moment of being photographed, although it had been pulled out of the water a little less than 3 min.

[Illustration:  PLATE XXVI, FIG. 1.—­TYPES OF ARCH TIMBERS USED IN BAY RIDGE TUNNEL SEWER.]

[Illustration:  PLATE XXVI, FIG. 2.—­NORMAL SLOPE OF LOOSE SAND, GRAVEL, AND CEMENTED GRAVEL, IN CLOSE PROXIMITY.]