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26. 

frictional resistance was experienced when the mass [[underlined]] M [[/underlined]] was raised by hand, provided the axis of the gun was kept strictly vertical, but a very considerable resistance was experienced if the axis was inclined to one side so that the holders [[underlined]] H [[/underlined]], [[underlined]] H' [[/underlined]] rubbed against the rods [[underlined]] R [[/underlined]], [[underlined]] R [[/underlined]]. This sideways pressure did not take place when the spring was used. It was also found that the trace upon the smoked glass was always slightly sinuous, with direct lift, and straight with the spring. The simple harmonic motion was, therefore, much the more preferable, but could not be used when the powder charges were large.

[[underlined]] MEANS OF ELIMINATING GASEOUS REBOUND. [[/underlined]]

It should be remembered that the real object of the vacuum experiments is to ascertain what the reaction experienced by the chamber would be, if a given charge of powder were fired in the chamber many miles above the earth's surface. A container is therefore necessary, which, for the purpose at hand, approaches most nearly a container of unlimited capacity. A length of 3-inch pipe, closed at the ends is evidently unsuitable, because the gas, fired from one end, is sure to rebound from the other end with considerable velocity, and hence produce a much larger displacement than ought really to be observed. Moreover, any tank of finite size must necessarily produce a finite amount of rebound, from the fact that the whole action is equivalent to liberating suddenly, in the tank, one or two liters of gas at atmospheric pressure.

There are two possible methods for reducing the velocity of the gas sufficiently to produce a negligible rebound: a [[underlined]] disintegration method [[/underlined]], whereby the stream is broken up into many small