228 Mr. F. J. Bramwell [June 13, 



diminislied distance, and for a correspondingly diminished time. Take, 

 for example, the force arising from the combustion of gunpowder, 

 and applied to the projectile through the space which that projectile 

 traverses in the bore of a cannon, and applied during the short time 

 in which that traverse is made. 



I have already told you that the length of the 38-ton bore is 

 16 feet 6 inches, but from this must be deducted, say, 2 feet for the 

 length of the cartridge, leaving about 14^ feet as the distance through 

 which the shot moves under the influence of the powder pressure. 

 Now 14J feet is the g^L g^th part of 30,000 feet ; therefore the average 

 pressure on a projectile while traversing this 14J feet must be 

 2069 times as great as the weight of the shot, in order to give it as 

 great a velocity, 1400 feet in a second, as it would have attained by 

 falling 30,000 feet. I have said that this must be the average 

 pressure. Obviously as regards strain tending to burst the gun, the 

 most favourable condition of things would be that this average pressure 

 should be exercised, but, equally obviously, the explosion of a charge 

 of powder is not a means by which such an average can be attained. 

 To put it popularly, one feels that the explosion of the charge 

 in the small space which it occupies between the rear end of the 

 tube and the base of the projectile before that projectile begins to 

 move, must give rise to an intense pressure which will gradually 

 die out as, owing to the progression of the shot, the space becomes 

 enlarged, and as the gases are cooled. This being so, and 2069 

 times the weight of the shot being needed as the average pressure, it 

 follows that as the final pressure falls much below the average, the 

 commencing pressure must be greatly above it ; and it is this great 

 commencing pressure which strains the gun and demands the enor- 

 mous thicknesses of metal which you see surrounding the powder 

 chamber. 



I believe I have now, by what I fear has been too long an introduc- 

 tion, prepared the way to show you why it is that the large-grain slow- 

 burning powders tax the resisting power of the gun less than it is taxed 

 by the small-grain and quick-burning, for it will readily occur to you 

 that if the chamber be occupied by large cubes of powder which 

 begin burning from the outside, and in this beginning of burning 

 generate a certain pressure, the shot will commence to move, and the 

 space in which the powder gases are confined will commence to in- 

 crease, so that by the time the whole of the powder is ignited the 

 chamber will have become larger, and thereby the intensity of the 

 pressure will bo reduced ; while if the powder had been composed of 

 small grains, like those used in a rifle, the explosion of the charge 

 would take place in so short a time that the shot would not have 

 appreciably moved before the whole of the charge was in combustion, 

 and in this way a very high pressure would be produced. I may 

 mention that the French apply to this small-sized powder when used 

 in cannon the expressive title " poudre brutale." 



Diagram 6 is a " curve of pressures." Imagine that the horizontal 



