The fact that small cavities may be excavated from the sides of trenches or tunnels back of the sheeting proves only that there is a local temporary arching of the material, or that the cohesion of the particles is sufficient to withstand the stress temporarily, or that there is a combination of cohesion and arching.  The possibility of making such excavations does not prove that pressure does not exist at such points.  That sand or earth will arch under certain conditions has long been an accepted fact.  The sand arches experimented with developed their strength only after considerable yielding and, therefore, give no index of the distribution or intensity of stress before such yielding.  Furthermore, sand and earth in Nature are not constrained by forms and reinforcing rods.

Mr. Meem’s paper is very valuable in that it presents some unusual phenomena, but many of the conclusions drawn therefrom cannot be accepted without further demonstration.

FRANCIS W. PERRY, ASSOC.  M. AM.  SOC.  C. E.—­Pressure-gauge observations on a number of pneumatic caissons recently sunk, through various grades of sand, to rock at depths of from 85 to 105 ft. below ground-water, invariably showed working-chamber air-pressures equal, as closely as could be observed, to the hydrostatic pressures computed, for corresponding depths of cutting-edge, as given in Table 2.

These observations and computations were made by the speaker in connection with the caisson foundations for the Municipal Building, New York City.

TABLE 2.—­EQUIVALENT FEET OF DEPTH BELOW WATER PER POUND PRESSURE.

Pressure, |Equivalent |Equivalent   |Observed      |
in        |feet of    |elevation    |pressure.     |
pounds.   |depth.     |for water    |              |
|           |at--6.85.    |              |
|___________|_____________|              |
|           |             |              |
|M.H.W.     |Ground-water.|              |
__________|___________|_____________|______________|
|           |             |              |
1       |  2.31     |  9.06       |Practically   |
2       |  4.63     | 11.48       |the same as   |
3       |  6.94     | 13.79       |computed      |
4       |  9.25     | 16.10       |for           |
5       | 11.57     | 18.42       |ground-water. |
6       | 13.88     | 20.73       |              |
7       | 16.19     | 23.04       |              |
8       | 18.50     | 25.35       |              |
9       | 20.82     | 27.67       |              |
10       | 23.13     | 29.98       |              |
11       | 25.44     | 32.29       |              |
12       | 27.76     | 34.61       |              |
13       | 30.07     | 36.92       |              |
14       | 32.38     | 39.23       |              |
15       | 34.70     | 41.55       |              |
16       | 37.01     | 43.86       |              |
17       | 39.32     | 46.17       |              |
18       | 41.63     | 48.48       |              |