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30. 

the 3-inch pipe [[underline]] P [[/underline]] by this trap, which would be sprung at the instant of firing. In this way gaseous rebound would be entirely eliminated.  It was found, however, that results with the two methods already described could be checked sufficiently to make this modification unnecessary. 

The tanks were exhausted by a rotary oil pump, No. 1, of the American Rotary Pump Company, supported by a water jet pump. In this way the pressure in the cylindrical tank could be reduced to 1.5 mm. of mercury in 25 minutes and to the same pressure in the circular tank, in 10 minutes. The pressures employed in the experiments ranged from 7.5 mm. to 0.5 mm.

[[underline]] METHODS OF DETECTING AND MEASURING GASEOUS REBOUND [[/underline]].

With the two tanks used in the experiments, it was obviously impossible to eliminate gaseous rebound entirely, from the fact that, even if the velocity of the gases is reduced to zero, there still remains the effect of introducing suddenly a certain quantity of gas into the tank.  It became necessary, then, to devise some means of detecting, and, if possible, of measuring, the extent of the rebound.

Three devices were employed, one for detecting a [[underline]] force [[/underline]] of rebound, and two for measuring the magnitude of the [[underline]] impulse per unit area [[/underline]] produced by the rebounding gas. These latter devices from the fact that quantitative measurements were possible with them, will be called "impulse-meters".