i817.] On Explosions in Coal-Mines. 133 
In describing the reflected explosive motion I omitted to mention 
an important circumstance. When the explosion has struck one 
side of a working at an oblique angle, the air in motion leaves it at 
an angle which is a little less than that by which it approached the 
side. This I have shown before; but that part of the air which 
approached the working on the left hand of the explosion leaves the 
side on the right hand: hence an internal whirling motion is im- 
parted to the blast every time it strikes the coal wall. This whirling 
motion collects the dust, and other light bodies, which the miners 
find floating in the air, on entering a mine soon after an explosion : 
and it is this motion that tosses the miner after he has felt the first 
contact ; and which very often does him more injury, by bringing 
sharp stones or loose pieces of wood against him; and sometimes 
lifts him from the ground, and enables the direct motion to carry 
him forward against the sides of the working, or against pillars of 
fallen roof, props, or other exposed bodies. The undisturbed ex- 
plosion, though pretty strong, does him very little comparative in- 
jury, if it gives him time to lie down, as it generally does by its 
thunder-like noise ; for then it passes by without displacing him ; 
and he has only to fear being injured by small loose pieces of wood 
or stone that it may possibly bring along with it. 
The motion towards the place of combustion may happen under 
different circumstances. When the explosion reaches a forehead it 
rebounds again. When a great quantity of heat is suddenly given 
out by combustion in a confined place, a series of rapid retrograde 
movements is made in the immediate vicinity of the fire, until an 
adequate degree of pressure is obtained to force out the displaced 
air. The nature of this retrograde motion, which I have before 
described, may be illustrated by the following example. Suppose 
a sluice, having in its middle a small upright oblong aperture, which 
reaches from the top to the bottom, be put into a descending trough, 
through which there runs a body of water. The sluice prevents the 
most of the water from passing ; but the water kept back obtains, 
in the following manner, a sufficient altitude to force itself through 
the aperture. It returns from the sluice with a small head, or a 
level, until it is lost in the ascent of the trough; this is no sooner 
done than a similar backward current commences at the sluice, and 
retires until it meets the descending current of water. These back- 
ward currents are repeated until the head obtained is sufficient to 
force the water through the aperture; but before they cease, too 
great an altitude is usually obtained, and then a few small currents 
run toward the sluice until the exact altitude is acquired. These 
currents of water are highly illustrative of the retrograde motions 
in coal-mines. Such motions in these mines also take place when 
the explosion has pervaded a great area of mine, if it be then pent 
in by a few narrow workings, as at the narrow openings g and B. 
Retrograde motions are also observed when the explosion is at first 
retarded by the doors and stops of ventilation; but they cease when 
it has obtained a sufficient pressure to displace these obstructions. 
