573 
where they now intermix or retreat, to the detriment and 
peril of every living creature in it. For however energetic 
the current may be, which is maintained in the cupola or 
up-cast shaft, upon the present system of ventilation by 
return-air-courses, the stimulus does but apply in the direction 
of the air-courses themselves ; and in such direction the 
energy is equally participated in by perfection and defects. 
Leakages from the right channel, as well as the proper 
current itself, receive accession of strength by the vigour 
imparted to the up-cast draft. But remembering the kindred 
relation of gases to fluid bodies, as regards obedience to 
certain laws of motion, we must at once recognise the energy 
due to their relative gravities ; and, herein, the repugnance 
with which a lighter medium is made obedient to the impulses 
given to a heavier one. Hence the perfect repose of fire- 
damp in the lateral cavities or " goafs" of a mine, whilst the 
current of atmospheric and heavier air is sweeping past along 
the air-courses, in the direction of the up-cast. In active 
motion, the two may be blended. But no natural affinity 
exists which would secure dilution of fire-damp by a voluntary 
admixture of this gas with the fresh or atmospheric air where 
no current existed. And thus may be accounted for the 
presence of fire-damp, forming an independent statum, even 
in the passages along which, but at a lower level, the air may 
be sweeping in a condition of perfect purity and safety. 
The practical lesson to be learnt from these facts is, — that 
along the return-courses of the existing system, which from 
the very nature of things implies a descending plane of 
communication, — no voluntary action can be relied on for 
inducing the escape of carburetted hydrogen or explosive 
gas. The consequence of which is, that the lighter and 
explosive gas, ever isolating itself from surrounding media, 
stealthily creeps into every cavity, away from draft or dis- 
turbance, until, by the fact of its separation and increase, it 
