368 



diminution of one-inch mercurial pressure, equal to 1*007 second, 

 which coincides almost exactly with the number (T043) deduced 

 from the author's experiments. 



The results of both series of observations may therefore be em- 

 bodied in the following law : The increments in time are propor- 

 tional to the decrements in pressure. For all practical purposes 

 the following rule may be adopted : Each diminution of one inch of 

 barometrical pressure causes a retardation of one second in a thirty- 

 seconds fuse ; or, each diminution of atmospheric pressure to the 

 extent of one mercurial inch increases the time of burning by one- 

 thirtieth. 



This retardation in the burning of time-fuses by the reduction of 

 atmospheric pressure will probably merit the attention of artillery 

 officers. Up to the present moment these fuses have been carefully 

 prepared so as to burn, at Woolwich, a certain number of seconds ; 

 but such time of combustion at the sea-level is no longer maintained 

 when the fuses are used in more elevated localities. Even the ordi- 

 nary fluctuations of the barometer in our latitude must render the 

 time of the combustion of these fuses liable to a variation of about 

 ten per cent. Thus a fuse driven to burn thirty seconds when the 

 barometer stands at 31 inches, would burn thirty-three seconds if the 

 barometer fell to 28 inches. Even the height to which a shell 

 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 

 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 



