Air enters below to supply its place, is rarefied in its turn, and rises; and this Operation continues till the Funnel grows cooler, or the outward Air warmer, or both, when the Motion ceases. On the other Hand, if after a cold Season, the outward Air suddenly grows warm and of course lighter, the Air contain'd in the cool Funnels, being heavier, descends into the Room; and the warmer Air which enters their Tops, being cool'd in its turn and made heavier, continues to descend; and this Operation goes on, till the Funnels are warmed by the Passing of warm Air thro' them, or the Air itself grows cooler. When the Temperature of the Air and of the Funnels. is nearly equal, the difference of Warmth in the Air between Day and Night is sufficient to produce these Currents, the Air will begin to ascend the Funnels as the Cool of the Evening comes on, and this Current will continue till perhaps 9 or 10 o'clock the next Morning, when it begins to hesitate; and as the heat of the Day approaches, it sets downwards, and continues so till towards Evening, when it again hesitates for some time, and then goes upwards constantly during the Night, as before mentioned. Now when Smoke issuing from the Tops of neighbouring Funnels passes over the Tops of Funnels, which are at the Time drawing downwards, as they often are in the Middle part of the Day, such Smoke is of necessity drawn into these Funnels, and descends with the Air into the Chamber.

The remedy is to have a Sliding Plate, hereafter describ'd, that will shut perfectly the offending Funnel.

9. Chimneys, which generally draw well, do nevertheless sometimes give Smoke into the Rooms, it being driven down by strong Winds passing over the Tops of their Funnels, tho' not descending from any commanding Eminence. This Case

is most frequent where the Funnel is short, and the Opening turn'd from the Wind. It is the more grievous, when i happens to be a cold Wind that produces the Effect, because when you most want your Fire, you are sometimes oblig'd to extinguish it. To understand this, it may be consider'd that the rising light Air, to obtain a free Issue from the Funnel, must push out of its Way or oblige the Air that is over it to rise. In a time of Calm or of little Wind this is done visibly, for we see the Smoke that is brought up by that Air rise in a Column above the Chimney. But when a violent Current of Air, that is, a strong Wind, passes over the Top of a Chimney, its Particles have receiv'd so much Force, which keeps them in a horizontal Direction, and follow each other so rapidly, that the rising light Air has not Strength sufficient to oblige them to quit that Direction and move upwards to permit its Issue. Add to this, that some of the Current passing over that Side of the Funnel which it first meets with, viz. at A, (Plate, Fig. 5,) having been compress'd by the Resistance of the Funnel, may expand itself over the Flue, and strike the interior opposite Side at B, from whence it may be reflected downwards and from Side to Side in the Direction of the prickt Line c c c.

Remedies. In some Places, particularly in Venice, where they have not Stacks of Chimneys but single Flues, the Custom is, to open or widen the Top of the Flue, rounding in the true Form of a Funnel; (Plate, Fig. 6;) which some think may prevent the Effect just mentioned, for that the Wind blowing over one of the Edges into the Funnel, may be slanted. out again on the other Side by its Form. I have had no Experience of this; but I have lived in a Windy Country, where the contrary is practis'd, the Tops of the Flues being