RECENT EXPERIMENTS ON FOG SIGNALS. 197 



demonstrated, seemed to single it out as a substance emi- 

 nently calculated to fulfill the conditions necessary to the 

 production of an intense wave of sound. What those con- 

 ditions 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 



FIG. 6. 



Breech-loading Fog-signal Gun. with Bell Mouth,* proposed by 

 Major Maitland, R.A., Assistant Superintendent. 



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 distances through the atmosphere, and this requires 

 a certain choice and management of the explosive ma- 

 terial. 



* The carriage of this gun Las been modified in construction since 

 this drawing was made. 



