"A chimney may not only be defective by having the mantle too high, or by being too wide from side to side, but also by being too deep between the fore-side and the back, as is often the case in very old houses. In this case, the distance between the fire and the mantle is so great, that much air passes up without being sufficiently rarefied... This fault may be sometimes cured, by bringing the grate a little forward, which, by making the fire act more powerfully upon the mantle, rarefies the air more in its passage."