246 Prof. Tyndall. Experiments on Fog- Signals. [Mar. 21, 



The result of subsequent trial, as reported by General Campbell, is, 

 " that the sonorous qualities of bronze are greatly superior to those of 

 cast-iron at short distances, but that the advantage lies with the baser 

 metal at long ranges."* 



Coincident with these early trials of guns at Woolwich gun- 

 cotton was thought of as a probably effective sound-producer. 

 Theoretic considerations caused me to fix my attention persistently 

 on this substance ; for the remarkable experiments of Mr. Abel, 

 whereby its rapidity of combustion and violently explosive energy are 

 demonstrated, seemed to single it out as a substance eminently cal- 

 culated to fulfil the conditions necessary to the production of an intense 

 wave of sound. What those conditions are we shall now more par- 

 ticularly 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. The work 

 which the artillerist requires the expanding gas to perform is the 

 displacement of the projectile. Such, however, is not the work that 

 we want our gunpowder to perform. We wish it to transmute its 

 energy not into the mere mechanical translation of the shot, 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 pulse 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 pulse is not produced. Con- 

 sider the case of a common clock pendulum, which oscillates to and 

 fro, and which therefore might be expected to generate corresponding 

 pulses in the air. When, for example, the bob -moves to the right, 

 the air to the right of it might be supposed to be condensed, while 

 a partial vacuum might be supposed to follow the bob. As a matter 

 of fact, we have nothing of this kind. The air particles in front of 

 the bob retreat so rapidly, and those behind it close so rapidly in, that 

 no sound-pulse is formed. A tuning-fork which executes 256 com- 

 plete vibrations in a second, if struck gently on a pad and held in free 

 air, emits a scarcely audible note. It behaves to some extent like the 

 pendulum bob. This feebleness is due to the prompt "reciprocating 

 flow " of the air between the incipient condensations and rarefactions, 

 whereby the formation of sound-pulses is forestalled. Stokes, how- 

 ever, has taught us that this flow may be intercepted by placing the 



* Greneral Campbell assigns a true cause for this difference. The ring of the bronze 

 gun represents so much energy withdrawn from the explosive force of the gun- 

 powder. Further experiments would, however, be needed to place the superiority 

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



