RECENT EXPERIMENTS ON FOG SIGNALS. 199 



powder (P.) (See annexed figures.) The charge in each 

 case amounted to 4-J Ibs. ; four 24-lb. howitzers being 

 employed to fire 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 



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F.G. L.G. R.L.G. 



Fio. 7. 



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 

 manner. 



