RECENT EXPERIMENTS ON FOG-SIGNALS. 201 



air, instead of the sharp shock necessary 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 rapidity sufficient to forestall the 

 formation of the wave. On a priori grounds then, we 

 are entitled to infer the effectiveness of gun-cotton, 

 while in a great number of comparative experiments, 

 stretching from 1874 to the present time, this infer- 

 ence has been verified in the most conclusive manner. 



As regards explosive material, and zealous and ac- 

 complished help in the use of it, the resources of 

 Woolwich Arsenal have been freely placed at the dis- 

 posal of the Elder Brethren. General Campbell, Gen- 

 eral Younghusband, Colonel Fraser, Colonel Maitland, 

 and other officers, have taken an active personal part in 

 the investigation, and in most cases have incurred the 

 labour of reducing and reporting on the observations. 

 Guns of various forms and sizes have been invoked for 

 gunpowder, while gun-cotton has been fired in free air 

 and in the foci of parabolic reflectors. 



On the 22nd of February, 1875, a number of small 

 guns, cast specially for the purpose some with plain, 

 some with conical, and some with parabolic muzzles 

 firing 4 oz. of fine grain powder, were pitted against 

 4 oz. of gun-cotton detonated both in the open, and in 

 the focus of a parabolic reflector.* The sound pro- 

 duced by the gun-cotton, reinforced by the reflector, 

 was unanimously pronounced loudest of all. With 



* For charges of this weight the reflector is of moderate size, 

 and rauy be employed without fear of fracture. 



