RECENT EXPERIMENTS ON FOG-SIGNALS. 313 



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 inference 

 has been verified in the most conclusive manner. 



As regards explosive material, and zealous and 

 accomplished 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, Gene- 

 ral 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 agninst 

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

 the focus of a parabolic reflector. 1 The sound produced 

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

 unanimously pronounced loudest of all. With equal 



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

 and may be employed without fear of fracture. 



