MECHANICS AND USEFUL ARTS. 91 







a rapidly-burning powder, to make the gun comparatively very thick 

 from the breech towards the trunnions ; and the production of strong 

 cast-iron guns of this form is attended with very considerable difficul- 

 ties ; but if we use a slower powder, we can employ a cannon of 

 more uniform thickness, as the strain exerted by the exploding 

 powder is distributed much more uniformly throughout the greater 

 part, if not the whole, of the length of the gun. Again, in rifled 

 guns, in which, in consequence of the accurate fit of the projectile, 

 the friction between it and the bore of the gun is very great, in 

 some of which, also, as in Sir William Armstrong's gun, the projec- 

 tile has to be expanded by the explosion into the grooves of the can- 

 non, a gradually progressive action of the explosion is obviously of 

 very great importance. In mortars and very short guns, on the other 

 hand, where we have a very small space for the projection of the 

 shell, it is necessary to employ a very rapidly-burning powder. It 

 has been constantly observed that, in firing mortars with the descrip- 

 tion of powder used for cannon, a portion of the charge has been 

 thrown out unexploded. 



The explosive action of gunpowder may, however, be easily regu- 

 lated in a variety of ways. We may do so, for example, by altering 

 the composition of the powder. By increasing the proportion of the 

 charcoal beyond that indicated by theory as the smallest quantity 

 which will combine with the whole of the oxygen in the saltpetre, 

 we decrease the rapidity of burning of the powder, simply because 

 we give the saltpetre a larger amount of carbon to oxidize, which it 

 wih 1 do less energetically ; the oxidation will, therefore, take place less 

 rapidly, and less heat will be developed. Then, again, we can mod- 

 ify the rate of combustion of the powder by altering the method of 

 manufacture, that is, by rendering the mixture of ingredients 

 more or less perfect. And, lastly, we may modify the action of pow- 

 der and this is really the correct way of going to work without 

 interfering at all with the proportions of ingredients as indicated by 

 theory, or with the intimacy of mixture essential to their perfect 

 action, by simply modifying the state of division of the material ; 

 that is to say, by employing various sizes of grains of gunpowder, 

 and also by submitting it to different degrees of compression. A 

 comparison of the rate of burning of two or three different samples of 

 powder of the same composition, but varying in size of grain, will show 

 you that this modification in the rapidity of action of gunpowder may 

 be effected with very great ease and nicety without interfering in 

 any way with the ultimate amount of force exerted by the powder. 

 Large grains, or rather lumps, have been tried, and, by employing 

 them judiciously, it is found that they propel shot to an equal dis- 

 tance, and with even greater uniformity, than ordinary cannon 

 powder. By combining the application of uniform and accurately 

 regulated pressure with modifications in the composition of gun- 

 powder, and by confining the material within a case or receptacle, so 

 that, when ignited, it can only burn in one direction, the valuable 

 arrangements known as fuses and time-fuses are obtained, by which 

 charges of powder in shells may be ignited at any period, within cer- 

 tain limits, determined upon previously to the loading of the cannon. 

 By mechanical arrangements, which regulate the amount of com- 



