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 has been modified in construction since 
this drawing was made. 
