1816.] On the Ventilation of Coal-Mines. 297 
downcast pit, called the first of the air; while they may be safely 
entered with any description of lights near the upcast pit, called 
the last of the air.* ‘The first of the air is the most dense, and as 
fluids are known to propagate pressure equally in every direction, it 
communicates its own degree of density to the fire-damp, with 
which it may meet in the neighbourhood of the downcast pit, thus 
fulfilling one of the conditions of explosion. This density will 
diminish asthe mixture rises to the upcast shaft; it will also be 
diminished by the increase of temperature which it will gradually 
acquire by the way, and the air will consequently be deprived in a 
corresponding degree of the property of exploding. Until these, 
and other facts, can be otherwise explained, it may be taken for 
granted, that the deepest part of a coal-mine, or that part of it 
where the inflammable mixture is more forcibly compressed, is the 
most dangerous, and that of any number of mines, of unequal 
depths, affording the same quantities of fire-damp, the deepest 
must be the most dangerous. This discussion may close here, with 
a quotation from a memoir by Grotthuss, which appeared in the 
Annales de Chimie, for April, 1812. It is peculiarly applicable, 
and may help to deter anonymous writers from hazarding unsup- 
ported opinions on the subject. Having, by the most satisfactory 
experiments ascertained the fact, that gaseous mixtures are more 
or less inflammable as the pressure upon them varies; ‘ D’aprés 
cela,” says he (page 41,) ‘ ilest trés possible, qu’ au fond des mines - 
de sel a Cracovie, 4 Amsterdam, ou dans un autre ville basse, un 
mélange gazeux pourrait étre enflammé, tandis qu’il ne serait plus 
inflammable, dans une ville tres-eleyée comme a Quito dans l’Ame- 
rique Meridionale.” 
There is yet another cause of compression, which must produce 
effects proportioned to its intensity upon every species of air which 
is found in coal-mines. ‘This is the foree, by which the current of 
air is induced, or accelerated, through the workings, from one end 
of amine to the other; whether by exhaustion, or by the more 
common mode of rarefaction by the application of heat. The 
variations of atmospheric compression are wholly beyond our con- 
‘troul; but it seems probable, that the other species of force might 
be so regulated in its application, as to balance in some degree the 
effects of those variations. When the barometer sinks, vast quan- 
titities of gas are liberated from every perpendicular fissure in the 
roof, in which it has been confined by the superior weight of the 
air. 
Were the downcast shaft occasionally closed, a similar effect 
would follow, from the suspension of the force which induces the 
atmospheric current through the mine. A quantity of gas pro- 
portionate to the degree of force which held it in confinement, 
would be thus set at liberty, and would find its way to the upper 
* See a letter by Mr. John Buddle, in the first Report of the Society for pre- 
venting Accidents in Coal Mines, page 20. 
Vor. VII, N° IV. 
