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19.

the outer boundary being indicated by dotted lines, [[underline]] P [[/underline]] being the powder, and [[underline]] W [[/underline]] the wadding.  It will be noticed that the wadding is just twice the size of that previously used.  

The mounting of the chamber, for the experiments, is shown in Fig. 13.  The chamber was held in the lower end of a 3-1/2 foot length of 2-inch pipe, [[underline]] P [[/underline]] by set-screws.  Within this pipe, above the chamber, was fastened a length of 2-inch steel shafting, to increase the mass of the movable system.  This system was supported by a half-inch steel pin, [[underline]] E [[/underline]].

On firing, the recoil lifted the above system vertically upward against gravity; the extent of the lift, or displacement, being recorded by a thin lead pencil, slidable in a brass sleeve set in the pipe at right angles to the pin [[underline]] E [[/underline]].  The point of the pencil was pressed against a vertical cardboard, [[underline]] C [[/underline]], by the action of a small spring.  This method of measuring the impulse of the expelled gases will be called the "direct-lift" method; and the theory is given in Appendix [[underline]] A [[/underline]], p.72.  

Although rebound of the gases from the ground would probably have been negligible, such rebo^[[u]]nd was eliminated by a short plank,[[underline]] D [[/underline]] covered with a piece of heavy sheet iron, and supported at an angle of 45° with the horizontal. This served to deflect the gases to one side.

The results of two experiments, 51 and 52, with this large chamber, are given in Table II.  In experiment 51, with Du Pont powder, the powder was packed rather loosely.  Any increase in internal diameter was inappreciable;  certainly under 0.01 mm.  In experiment 52, the Infallible powder was somewhat compressed.  After firing, the chamber was found to be slightly bulged for a short distance around