634 DE. E. EEANKLAlfD ON THE INELTJENCE OF ATMOSPHEEIC PEESSURE 
times of combustion by two, in order to bring them into comparison with the six-inch 
fuses which were used in my experiments : — • 
Pressiu'e in inches of 
mercury. 
Average time of com- 
bustion of 6-inch 
fuse. 
Increase of time of 
combustion over last 
observation. 
Reduction of pressure 
corresponding to in- 
crease of time. 
Increase of time for 
each diminution of 
1 inch pressure. 
29-61 
seconds. 
28-50 
seconds. 
inches. 
seconds. 
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 fuse, equal to 1’007 second for each diminution of one- 
inch mercurial pressure, which coincides almost exactly with the number (1-043) deduced 
from my own experiments. 
The results of both series of observations may therefore be embodied in the following 
law: — The increments in time are proportional to the decrements in pressure. 
For all practical purposes the following rule may be adopted : — Each diminution of 
one inch of larometrical pressure causes a retardation of one second in a six-inch or 
thirty-second 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. These fuses have ' hitherto been 
carefully prepared so as to burn, at Woolwich, a certain number of seconds, and the 
perfection with which this is attained is highly remarkable ; but such time of com- 
bustion at the sea-level is no longer maintained wffien the fuses are used in more 
elevated localities. The ordinary fluctuations of the barometer in our latitude must 
render the rate of combustion of these fuses liable to a variation of about ten per 
cent. Thus a fuse driven to burn 30 seconds when the barometer stands at 31 inches, 
would bum 33 seconds if the barometer fell to 28 inches. The height to which a 
shell attains in its flight must exert an appreciable influence upon the burning of its 
time fuse ; to a far greater extent, however, must the time of combustion be affected 
by the position of the fuse during the flight of a rifled shell : as in these projectiles the 
fuse always precedes the shell, the time of burning must obviously be very much shorter 
than w-hen the shell and fuse are at rest. In an ordinary shell which rotates upon a 
horizontal axis, the alternate compression and rarefaction of the air at the mouth of the 
fuse, although tending to compensate each other, will still leave a considerable balance 
of compression, which must cause a marked retardation in the rate of burning. 
The apparently opposite conclusions to w'hich 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-fuses on the other, are by no means irreconcileable ; in 
