THE ROYAL ARTILLERY INSTITUTION. 
15 
multiplied by two in order to bring them into comparison with the six-inch 
fuzes which were used in the author’s experiments:— 
Pressure of 
air in inches 
of mercury. 
Average time 
of combustion 
of 6-in. fuze. 
Increase of 
time of 
combustion 
over last 
observation. 
Reduction of 
pressure 
corresponding 
to increase 
of time. 
Increase of 
time for each 
diminution of 
1-in. pressure. 
seconds. 
seconds. 
inches. 
seconds. 
29-61 
28-50 
26-75 
31-56 
3-06 
2-86 
1-070 
23-95 
34-20 
2-64 
2-80 
•943 
22.98 
36-25 
2-05 
•97 
2-113 
Here, omitting the last determination as abnormal, we have the average 
retardation, in the combustion of a six-inch fuze, for each diminution of 
one-inch mercurial pressure, equal to 1*007 second, which coincides almost 
exactly with the number (1*043) deduced from the author’s experiments. 
The results of both series of observations may therefore be embodied in 
the following law :— The increments in time are proportional to the decre¬ 
ments in pressure. Tor 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 fuze ; 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-fuzes by the reduction of atmo¬ 
spheric pressure will probably merit the attention of artillery officers. Up to 
the present moment these fuzes 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the sea-level is no longer maintained when the fuzes are used in more 
elevated localities. Even the ordinary fluctuations of the barometer in our 
latitude must render the time of the combustion of these fuzes liable to a 
variation of about ten per cent. Thus a fuze 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-fuze; to a still greater extent, however, must the time of combustion 
be affected by the position of the fuze 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 comparatively 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 combustion, by the 
experiments upon candles on the one hand and upon time-fuzes 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 
