258 FRAGMENTS OF SCIENCE. 



Coincident with these trials of guns at Woolwich, 

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

 producer. From the first, indeed, theoretic considera- 

 tions caused me to fix my attention persistently on this 

 substance ; for the remarkable experiments of Mr. Abel, 

 whereby its rapidity of combustion and violently ex- 

 plosive 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 enquire, 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 in- 

 tensely heated gas. Now the work which the artillerist 

 requires the expanding gas to perform is the displace- 

 ment 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 per- 

 form. 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 distances 

 through the atmosphere, and this requires a certain 

 choice and management of the explosive material. 



A sound-wave consists essentially of two parts a 

 condensation and a rarefaction. Now air is a very 

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

 promptness, the wave is not produced. Consider the 

 case of a common clock pendulum, which oscillates to 



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. 



