On the Influence of Atmospheric Pressure upon Combustion. 233 



There are here evident indications of the rate of retardation being 

 somewhat greater at low than at comparatively high pressures ; but, 

 neglecting these indications,, the above numbers give T043 second 

 as the average retardation in a six-inch or thirty-seconds fuse for 

 each inch of mercurial pressure removed. This result agrees closely 

 with that obtained by Quartermaster Mitchell, if we except those 

 fuses which he burnt at the greatest altitude ; and in reference to 

 which some error must obviously have crept in. The following 

 Table shows Mr. Mitchell's results uniformly with those in the last 

 Table. The fuses which he employed were fifteen-seconds or three- 

 inch ones, and their times of combustion have therefore been multi- 

 plied by two in order to bring them into comparison with the six- 

 inch fuses which were used in the author's experiments : — 



Pressure of air 



in inches of 



mercury. 



Average time 



of combustion 



of six-inch 



fuse. 



Increase of 

 time of com- 

 bustion over 

 last observa- 

 tion. 



Reduction of 

 pressure cor- 

 responding to 

 increase of 

 time. 



Increase of 



time for each I 



diminution of 



one-inch 



pressure. 



29-61 

 26-75 

 23-95 

 22-98 



Seconds. 

 28-50 

 31-56 

 34-20 

 36-25 



Seconds. 



3-05 

 2-64 

 2-05 



Inches. 



2-86 



2-80 



•97 



Seconds. 



1-070 



•943 



2-113 



Here, omitting the last determination as abnormal, we have the 

 average retardation, in the combustion of a six-inch fuse, 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 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 j)ressure 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 Yv r oolwich, 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 



Phil. Mag. S. 4. Vol. 24. No. 1G0. Sept 18G2. R 



