RECENT EXPERIMENTS ON FOG-SIGNALS 273 



Coincident with these trials of guns at Woolwich, gun- 

 cotton was thought of as a probably effective sound-pro- 

 ducer. From the first, indeed, theoretic considerations 

 caused me to fix my attention persistently on this sub- 

 stance; for the remarkable experiments of Mr. Abel, 

 whereby its rapidity of combustion and violently explo- 

 sive energy are demonstrated, seemed to single it out as 

 a substance eminently calculated to fulfil the conditions 

 necessary to the production of an intense wave of sound. 

 What those conditions are we shall now more particularly 

 inquire, calling to our aid a brief but very remarkable 

 paper, published by Professor Stokes in the "Philosophical 

 Magazine" for 1868. 



The explosive force of gunpowder is known to depend 

 on the sudden conversion of a solid body into an intensely 

 heated gas. Now the work which the artillerist requires 

 the expanding gas to perform is the displacement of the 

 projectile, besides which it has to displace the air in front 

 of the projectile, which is backed by the whole pressure of 

 the atmosphere. Such, however, is not the work that we 

 want our gunpowder to perform. We wish to transmute 

 its energy not into the mere mechanical translation of either 

 shot or air, but into vibratory motion. We want pulses to 

 be formed which shall propagate themselves to vast dis- 

 tances through the atmosphere, and this requires a certain 

 choice and management of the explosive material. 



A sound-wave consists essentially of two parts a con- 

 densation and a rarefaction. Now, air is a very mobile 

 fluid, and if the shock imparted to it lack due promptness, 



bronze gun represents so much energy withdrawn from the explosive force of 

 the gunpowder. Further experiments would, however, be needed to place the 

 superiority of the cast-iron gun at a distance beyond question. 



