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38.

the tissue paper is sometimes torn and sometimes not under identical conditions of firing shows that either this force differs more or less in various parts of the tank (i.e., the upward rush of gas is not perfectly homogeneous); or that the tissue paper is weakened by each successive shot. This last explanation is the more probable; for fine particles of the wadding rush upward with the gas, as is proved by fine markings on the smoked glass, and also from the fact that, after a number of shots, the tissue paper is found to be perforated with very small holes.

The gaseous rebound could not be measured accurately with the direct-lift impulse-meter. Thus of all the experiments in which this meter could be used, 15 to 26 inclusive, only two, 16 and 22, gave readable displacements; the failure to obtain readable displacements in the other cases being doubtless due to friction, as already mentioned. It will be noticed that the impulse is under one per cent.

The spring impulse-meter, used in the last five experiments gave reliable results because of the very slight friction during operation. This impulse-meter shows that, if the momentum of the chamber were to be corrected for gaseous rebound, [[underlined]] this correction would be much less than one per cent of the momentum of the chamber [[/underlined]]. But, as has been stated above, the impulse of the rebound at the chamber must be less than that at the impulse-meter, from the fact that gases may pass readily behind the chamber, and exert a downward pressure, and also because of friction in the 3-inch pipe. The effect of gaseous rebound is therefore negligible, and no account of it has been taken in calculating the velocities and efficiencies.