FOR the same Reason, if a Fire be kindled in an open Place, the Heat thereof will rarify the next circumambient Air; and that which is more remote being heavier, will press every where, and in all Directions, upon the Air that is rarified, and drive it to the Fire; the Flame and Sparks whereof will, together with the rarified Air, ascend in a conic Form, like the Flame of a Candle, in a trembling Motion, as it is more or less acted upon by the Pressure of the cold Air: And the Reason why the Flame is more contracted at the Top than Bottom, is, that the Heat at Top being less intense, the next adjacent Air is less rarified, and the gross Air confines it more. THEREFORE, when a Fire is lighted in a Chimney, the Heat rarifies the Air over and next the Fire, and makes it rise in the Funnel, and the common Air in the Room immediately supplies its Place, is rarified in its turn, and rises also. THIS Motion being thus generated, is continued by small Inlets of Air, through the Doors and Windows of the Room; and the larger the Fire, the greater will be the current of Air through their Crevices. If the Doors and Windows are so well fitted in their Frames, that all the Inlets together cannot supply so much Air as is wanted to carry off the Smoke, it will then hang about the Fire, gradually diminish, and at length totally extinguish it. VARIOUS are the Improvements that have been made in the Construction of Chimneys, to increase the Degree of Heat, to prevent Smoke, and to save in the Article of Fuel. AND notwithstanding the many Attempts to remedy the Defects in one or other of these Respects, the same have hitherto come short of the End proposed. Take the Article of Smoke for Instance: No Builder of Character will pretend to insure all the Rooms in a new-built House from smoking, appears from this, that they generally at first finish the Chimney Tops with what they call Roundings, and if, upon Trial, those do not answer, they either Hovel, or fix Earthen Pots, like a hollow Cylinder, or plant Tin Tubes on the Tops, not much unlike Organ-Pipes inverted; all which Methods, not only spoil the Symmetry of the Building, but what is still worse, they often leave the Chimnies as Smoky as they were at first, after a considerable Sum has been spent on a Cure. IN order to remedy all, or some of the Inconveniences already mentioned, a great Variety of Chimnies, Stove-Grates, and Close-Stoves have been invented. I shall describe some of the principal ones that have fallen under my Observation; and shall endeavour, as I go along, to point out their Advantages and Defects. FIRST, Mons. _Gauger_ has described seven sorts of Chimnies, which, however, all agree in general as to the Construction and Disposition of the principal Parts.