"[W]hen the grate is brought forward, there is a great vacancy left between it and the back of the chimney, so... air passes under the grate, and ascends behind it very little rarefied; so... there will be as much lost in this way as will be gained in the other: and as there is not enough of heated air... to make the vapour ascend with rapidity, they are often choaked with thick fuliginous vapours hanging in them, almost in equilibrio with the rest of the atmosphere, so that the least puff of wind beats them down the chimney, and pushes the smoke into the room; whereas, when it is far back, it is driven down upon the hearth, and rises upwards again when the gust is over, and a great deal of it is catched within the mantle as it rises, which in the other case would have been dispersed through the room."