MECHANICS AND USEFUL AKTS. 93 



pounds, though inapplicable as substitutes for gunpowder on account 

 of their great sensitiveness to the effects of heat, have, in conse- 

 quence of this very quality, received important applications in numer- 

 ous ingenious contrivances for effecting the ignition of gunpowder. 

 Well-known instances of such applications are, the employment of 

 fulminate of mercury in percussion caps ; of a mixture of chlorate 

 of potassa and sulphide of antimony in arrangements for firing can- 

 non by percussion and by friction, and for exploding shells by per- 

 cussion or concussion ; and of the same mixture exploded at will by 

 being brought into contact with a drop of strong sulphuric acid, for 

 the ignition of submarine mines or of signals. 



Other mixtures, combining a high degree of explosiveness with 

 power of conducting electricity, have been successfully applied to the 

 simultaneous ignition of numerous charges of gunpowder by electri- 

 city of high tension ; by means of one of them, recently discovered, 

 many mines may be simultaneously discharged, even by the employ- 

 ment of small magneto-electric machines ; the necessity for the em- 

 ployment of voltaic arrangements in mining operations being thus 

 entirely dispensed with. 



One of the most highly explosive mixtures at present known, con- 

 sisting of chlorate of potassa and amorphous phosphorus, has been 

 most ingeniously applied by Sir William Armstrong to the ignition of 

 his time-fuses, and to the production of concussion and percussion 

 fuses, remarkable for the great ease with which they are exploded. 

 The above mixture may be ignited by the application of a gentle 

 heat, or by submission to moderate pressure : if it is made up into a 

 hard mass by mixture with a little shellac varnish, the friction result- 

 ing from the rapid insertion of .a pin's point into the material suffices 

 to ignite it, even when it is well covered with varnish. Thus, in 

 Armstrong's time-fuse, winch, when fixed in its place in the head of 

 the shell, cannot, like ordinary fuses employed in smooth-bore guns, 

 be ignited by the flame of the exploding charge of powder (as the 

 shell accurately fits the bore of the gun), the fuse composition is 

 inflamed immediately upon the firing of the gun, in the following 

 manner : A small quantity of the phosphorus mixture is deposited at 

 the bottom of a cylindrical cavity in the centre of the fuse, and 

 over it is fixed a small plug of metal, with a pin's point projecting 

 from its lower end. This plug is held in its place by a pin of soft 

 metal, which, by reason of the vis inertice of the plug, is broken when 

 the gun is fired, and the pin then instantly pierces the pellet of deto- 

 nating mixture, which by its ignition sets into action the time-fuse. 

 The distance between the pin's point and the phosphorus mixture, 

 before the explosion, is only one-tenth of an inch. This arrangement 

 exemplifies in a striking manner the delicacy of action which may be 

 obtained by a judicious combination of simple mechanical arrange- 

 ments and highly explosive materials. 



The variety of work accomplished by the explosion of a charge of 

 powder in an Armstrong gun loaded with a shell no less than five 

 distinct and important operations being thereby effected before the 

 shell leaves the gun affords a most interesting illustration of the 

 progress made in the application of explosives, and of the compara- 



