234 Royal Society :— 



attains in its flight must exert an appreciable influence upon the 

 burning of its time-fuse ; 'to a still greater extent, however, must the 

 time of combustion be affected by the position of the fuse during the 

 flight of the shell. If it precede the shell, the time of burning must 

 obviously be considerably shorter than if it follow in the compara- 

 tively vacuous space behind the shell. 



The apparently opposite conclusions to which we are led as 

 regards the influence of atmospheric pressure upon the rate of com- 

 bustion, by the experiments upon candles on the one hand and upon 

 time-fuses on the other, are by no means irreconcileable ; in fact, an 

 examination into the conditions of combustion in the two cases 3 

 scarcely leaves room for the expectation of any other result. In the 

 combustion of a candle, the author proves that, at all pressures, 

 there is a sufficient supply of melted combustible matter kept up at 

 the base of the exposed portion of the wick : the capillarity of the 

 latter is not affected by pressure; and as the temperature of the 

 flame is also proved to remain practically constant, effecting the 

 evaporation of the same amount of combustible matter under all 

 pressures, it follows that the rate of consumption of a candle must 

 be nearly or quite independent of the pressure of the surrounding 

 medium. In the deflagration of time-fuses, the conditions are obvi- 

 ously very different. Here the combustible matter never comes into 

 contact with atmospheric oxygen until it has been ejected from the 

 fuse-case. Unlike the candle, the composition contains within itself 

 the oxygen necessary for combustion, and a certain degree of heat 

 only is necessary to bring about chemical combination. If this heat 

 were applied simultaneously to every part of the fuse composition, 

 the whole would burn almost instantaneously. Under ordinary cir- 

 cumstances, however, the fuse burns only at a disk perpendicular to 

 its axis ; and the time occupied in its deflagration necessarily depends 

 upon the rapidity with which each successive layer of composition is 

 heated to the temperature at which chemical combination takes 

 place. This heat, necessary to deflagration, is evidently derived 

 from the products of the combustion of the immediately preceding 

 layer of composition ; and the amount of heat thus communicated to 

 the next unburnt layer must depend, in great measure, upon the 

 number of particles of these heated products which come into con- 

 tact with that layer. Now, as a large proportion of these products 

 are gaseous, it follows that, if the pressure of the surrounding medium 

 be reduced, the number of ignited gaseous particles in contact at 

 any one moment with the still-unignited disk of composition will also 

 be diminished. Hence the slower rate of deflagration in rarefied 

 air. 



With regard to the effect of atmospheric pressure upon the light of 

 gas-flames, the author thus expresses the conclusion arrived at :■ — 

 Of 100 units of light emitted by a gas-Jlame burning in air at a 

 pressure of 30 inches of mercury, 5*1 units are extinguished by each 

 reduction of one mercurial inch of atmospheric pressure. Hence the 

 decrease in illuminating power is directly proportional to the de- 

 crease in atmospheric pressure. 



