_Author_. To answer this objection, I will desire leave to give you my notion of those combustibles, which, in short, is this: that when your miners come into a close place, where there is no circulation of air to carry off the effluvia, or atoms constantly rising like fine dust in a powder-mill, they by knocking and working do increase to be very numerous, like to those loose particles in a powder-mill. But it is the work of some time for those vapours to come to perfection; for I have heard several experienced miners say, that it is common to avoid the danger of those vapours, by retiring as soon as they see the flame of their candles burn longer than ordinary, which may be, discerned sometimes long before the air is thick enough of this combustible matter to take fire at once, and, like gunpowder, to destroy all. I did hear one say, that from an inch and a half, once the flame of his candle did gradually increase to two feet long, and yet he escaped. Which makes it very plain, that stagnation of air is the sole cause of this inconvenience in mines, which may be totally prevented by a pipe going from the ash-pit of our furnace to any part of the mine liable to stagnation. For the air will press with great violence through the pipe into the fire, before the combustible matter can be ready to do any hurt, and passing through the fire, make way for fresh air to descend in the room of it. So that our fire, instead of blowing up of your works, is the best means that can be used to prevent so fatal an accident; and will likewise carry off all unwholesome vapours, damps, or steams, which may proceed from corruption of air, by stagnations or vapours arising from any poisonous earth or mineral. _Miner_. This notion of yours carries reason and demonstration along with it, which pleases me wonderfully. But, sir, is not your price too great for these engines of yours?