PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 387 



have been used on board ships in the American service, and used 

 with very good eifect when vessels were engaged in close action. 

 It is evident that such a curtain can only be used with a breech- 

 loading cannon, for it would be of very little use if the gunner 

 was unprotected while charging the cannon. The question what 

 will be the effect upon them of the recoil of the gun, and whether 

 any provision will have to be made to prevent their being shaken 

 to pieces, will be best determined b}^ the test of trial. 



Mr. Haskell suggested that the cap which closed the barrel did 

 not entirel}' prevent the air from passing the ball between the 

 bars. 



Mr. De Brame. — I will remedy that. 



Mr. C. W. Smith expressed the opinion that the quality of the 

 powder was an important element in answering the question. 

 The friction of the barrel, especially for expanding balls, affects 

 the result. The result of the elaborate series of experiments of 

 the British government was, that the most convenient length for 

 cannon is from seven to eleven times the diameter of the bore. 



Dr. Vanderweyde said that if the barrel is closed and too long, 

 the ball would be retarded for the last part of the distance by the 

 resisting pressure of the air in front of the ball becoming greater 

 than the continually diminishing force of the powder. In such a 

 case, making the barrel shorter, or opening the sides as in the 

 model exhibited, would allow the escape of the air in front of the 

 ball, and permit it to move freely from its inertia. This cannon 

 certainly gives us the advantage of the greater accuracy of a 

 very long barrel, without the disadvantage of the resistance of 

 the air. It would seem as if the force of the powder would be 

 expended through those openings ; but the experiments seem to 

 show that the powder burns too quickly for that. The friction, 

 of course, is less, for the ball flies along upon a kind of rail with 

 only enough friction to direct its course. 



Mr. Rowell had witnessed experiments with rifle barrels to 

 ascertain at what length the charge exerted the maximum force 

 upon the ball, and the result was a barrel twenty-two inches in 

 length was found to be the most effective. The remaining length 

 of the rifle barrel then is only useful to direct the ball, and might 

 be opened without injury. 



Dr. Vanderweyde suggested that it would be an interesting 

 experiment with this revolving gun to fire it with its maximum 



