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20.

the middle of the powder chamber; the inside diameter being increased from 2.6 cm. to 2.7 cm; and the outside diameter from 5.08 cm. to 5.14 cm. The efficiency (64.53 per cent) in experiment 51, and the velocity (7,987 ft/sec.) in experiment 52 were, respectively the highest obtained in any of the experiments. 

The conclusions to be drawn from these two experiments are; first, that large chambers can be operated, under proper conditions, without involving undue pressures; and secondly, that large chambers, even with comparatively short nozzles, are more efficient and give higher velocities than small chambers. 

It is obvious that large grains of powder should be used in large chambers if dangerous pressures are to be avoided. The bulging in experiment 52 is to be explained by the grains of powder being too small for a chamber of the size under consideration. It is possible, however, that pressures even as great as that developed in experiment 52 could be employed in practice provided the chamber were of "built-up" construction. A similar result might possibly be had if several shots had been fired, of successively increasing amounts of powder. The result of this would have been a hardening of the wall of the chamber by stretching. Such a phenomenon was observed with the soft steel chamber already described, which was distended by the first few shots of Infallible powder, but thereafter remained unchanged with loads as great as those first used. 

[[underline]] EXPERIMENTS IN VACUO [[/underline]].

[[underline]] INTRODUCTORY [[/underline]].

Having obtained average velocities of ejection up to nearly 8,000