RECENT EXPERIMENTS ON FOG SIGNALS. 109 
powder (P.) (See annexed figures.) The charge in each 
case amounted to 4lbs.; four 24-lb. howitzers being 
employed to tire the respective charges. There were 
eleven observers, all of whom, without a single dissentient, 
pronounced the sound of the fine-grain powder loudest of 
all. In the opinion of seven of the eleven the large-grain 
powder came next; seven also of the eleven placed the 
rifle large-grain third on the list; while they were again 
unanimous in pronouncing the pebble-powder the worst 
sound-producer. These differences are entirely due to 
differences in the rapidity of combustion. All who have 
witnessed the performance of the 80-ton gun must have 
F.G. L.G. 
been surprised at the mildness of its thunder. To avoid 
the strain resulting from quick combustion, the powder 
employed is composed of lumps far larger than those of 
the pebble-powder above referred to. In the long tube of 
the gun these lumps of solid matter gradually resolve them- 
selves into gas, which on issuing from the muzzle imparts 
a kind of push to the air, instead of the sharp shock nec- 
essary to form the condensation of an intensely sonorous 
wave. 
These are some of the physical reasons why gun-cotton 
might be regarded as a promising fog-signal. Firing it as 
we have been taught to do by Mr. Abel, its explosion is 
more rapid than that of gunpowder. In its case the air 
particles, alert as they are, will not, it might be presumed, 
be able to slip from condensation to rarefaction with a ra- 
pidity sufficient to forestall the formation of the wave. On 
a priori grounds then, we are entitled to infer the effective- 
ness of gun-cotton, while in a great number of comparative 
experiments, stretching from 1874 to the present time, 
this inference has been verified in the most conclusive 
