In one of your Letters, a little before I left France, you desire me to give you in Writing my Thoughts upon the Construction and Use of Chimmeys, a Subject you had sometimes heard me touch upon in Conversation. I embrace willingly this Leisure afforded by my present Situation to comply with your Request, as it will not only show my Regard to the Desire of a Friend, but may at the same time be of some Utility to others, the Doctrine of Chimneys appearing not to be as yet generally well understood, and Mistakes respecting them being attended with constant Inconvenience if not remedied, and with fruitless Expence if the true Remedies are mistaken.

Those who would be acquainted with this Subject should begin by considering on what Principles Smoke assends in any Chimney. At first many are apt to think that Smoke is in its Nature and of itself specifically lighter than Air, and arises in it for the same reason that Cork rises in Water. These see no Cause why Smoke should not rise in the Chimney, tho’ the Room be ever so close. Others think there is a Power in Chimneys to draw up the Smoke, and that there are different Forms of Chimneys which afford more or less of this Power. These amuse themselves with searching for the best Form. The equal Dimensions of a Funnel in its whole Length is not thought Artificial enough, and it is made for fancied Reasons, sometimes tapering and narrowing from below upwards, and sometimes the contrary, &c. &c. A simple Experiment or two may serve to give more correct Ideas. Having lit a Pipe of Tobacco, plung the stem to the Bottom of a Decanter half fill’d with cold Water; then putting a Rag over the Bowl, blow thor’ i and make the smoke descend in the Stem of the Pipe, from the End of which it will rise in Bubbles thro’ the Water; and being thus cool’d will not afterwards rise to go out thro’ the Neck of the Decanter, but remain spreading itself and resting on the Surface of the Water. This shows that Smoke is really heavier than Air, and that it is carried upwards only when attach’d to or acted upon by Air that is heated, and thereby rarified and rendered specifically lighter than the Air in its Neighbourhood.

Smoke being rarely seen but in company with heated Air, and its upward Motion begin visible tho’ that of the rarified Air that drives it is not so, has naturally given rise to the Error that Air is a Fluid which has Weight as well as others, tho’ about 800 times lighter than Water.

I need not explain to you, my learned Friend, what is meant by rarified Air; but if you make the public Use you propose of this Letter, it may fall into the Hands of some who are unacquainted with the Term and with the Thing. These then may be told, that Heat makes the Particiles of Air recede from each other and take up more Space, so that the same Weight of Air heated will have more Bulk, than equal Weight of cold Air which may surround it, and in that Case must rise, being forc’d upwards by such colder and heavier Air which presses to get under it and take its Place. That Air is so rarified or expanded by Heat, may be proved to their Comprehension by a lank blown Bladder, which laid before a Fire will soon swell, grow tight and burst.

Another Experiment may be to take a glass Tube about an Inch in Diameter, and 12 Inches long, open at both Ends and fixed upright and on Legs, so that it need not be handled, for the Hands might warm it. At the End of a Quill fastened 5 or 6 Inches of the finest light filaments of Silk, so that it may be held either above the upper End of the Tube, or under the Lower End, your warm Hand being at a distance by the length of the Quill. If there were any Motion of Air thro’ the Tube, it would manifest itself by its Effect on the Silk. But if the Tube and the Air in it are of the same Temperature with the surrounding Air, there will be no such Motion whatever may be the Form of the Tube, whether crooked or strit, narrow below and widening upwards, or the contrary, the Air in it will be quiescent. Warm the Tube, and you will find as long as it continues warm, a constant Current of Air entring below and passing up thro’ it, till discharg’d at the Top; because the Warmth of the Tube being communicated to the Air it contains, rarefies that Air and makes it lighter than the Air without, which therefore presses in below, forces it upwards, follows and takes its place, and is rarefied in its turn. And without warming the Tube, if you hold under it a Knob of hot Iron, the Air there by heated will rise and fill the Tube, going out at its Top, and this Motion in the Tube will continue as long as the Knob remains hot, because the Air entring the Tube below is heated and rarefied by passing near and over that Knob.

That this Motion is produc’d merely by the Difference of Specific Gravity between the Fluid within and that without the Tube, and not by any fancied Form of the Tube itself, may appear by plunging it into Water contain’d in a Glass Jar a foot deep, thro’ which such Motion might be seen. The Water within and without the Tube being of the same Specific Gravity, balance each other and both remain at rest. But take out the Tube, stop its Bottom with a Finger and fill it with Olive Oil which is lighter than Water, then stopping the Top, place it as before, its lower End under Water, its Top very little above. As long as you keep the Bottom stopt, the Fluids remain at rest, but the Moment it is unstopt, the heavier enters below, forces up the Lighter, and takes its Place. And the Motion then ceases merely because the new Fluid cannot be successively made lighter, as Air may be by a warm Tube.

In fact no Form of the Funnel of a Chimney has any Share in its Operation or Effect respecting Smoke, except its Height. The longer the Funnel, if erect, the greater its Force when fill’d with heated and rarefied Air, to draw in below, and drive up the Smoke, if one may, in compliance with Custom, use the Expression draw, when in fact it is the superior Weight of the surrounding Atmosphere that presses to enter the Funnel below, and so drives up before it the Smoke and warm Air it meets with in its Passage.

I have been the more particular in explaining these first Principles, because for want of clear Ideas respecting them, much fruitless Expence has been occasion’d; not only single Chimneys, but in some Instances within my Knowledge whole Stacks having been pulled down and rebuilt with Funnels of different Forms imagin’d more powerful in drawing Smoke, but having still the same Height and the same Opening below, having perform’d no better than their Predecessors.

What is it then which makes a Smoky Chimney, that is, a Chimney which instead of conveying up all the Smoke, discharges a Part of it into the Room, offending the Eyes and damaging the Furniture?

The Causes of this Effect, which have fallen under my Observation, amount to Nine, differing from each other, and therefore, requiring different Remedies.

1. Smoky Chimneys in a new House, are such frequently from mere Want of Air. The Workmanship of the Rooms being all good, and just out of the Workman’s Hand the Joints of the Boards of the Flooring, and of the Pannels of Wainscotting are all true and tight, the more so as the Walls, perhaps not yet throroughly dry, preserve a Dampness in the Air of the Room which keeps the wood-work swelled and close. The Doors and the Sashes, too, being work’d with Truth, shut with Exactness, so that the Room is as tight as a Snuff-Box, no Passage being left open for Air to enter except the Key hole, and even that is sometimes covered by a little dropping Shutter. Now if Smoke cannot rise but as connected with rarefied Air, and a Column of such Air, suppose it filling the Funnel, cannot rise, unless other Air be admitted to supply its place; and if, therefore, no Current of Air enter the Opening of the Chimney, there is nothing to prevent the Smoke’s coming out into the Room. If the Motion upwards of the Air in a Chimney that is freely supply’d be observed by the Rising of the Smoke or a Feather in it, and it be considered that in the Time such Feather takes in rising from the Fire to the Top of the Chimney, a Column of Air equal to the Content of the Funnel must be discharged, and an equal Quantity supply’d from the Room below, it will appear absolutely impossible that this Operation should do on, if the tight Room is kept shut; for were there any Force capable of drawing constantly so much Air out of it, it must soon be exhausted like the Receiver of an Air pump, and no Animal could live in it. Those therefore who stop every Crevice in a Room to prevent the Admission of fresh Air, and yet would have their Chimney carry up the Smoke, require Inconsistencies and expect Imposibilities. Yet under this Situation, I have seen the Owner of a new House, in Despair, and ready to sell it for much less than it cost, conceiving it uninhabitable, because not a Chimney in any one of its Rooms would carry off the Smoke, unless a Door or Window were left open. Much Expence has also been made, to alter and amend new Chimneys which had really no Fault; in one House particularly that I knew of a Nobleman in Westminster, that Expence amounted to no less than £300, after his House had been as he though finished and all Charges paid. And after all, several of the Alternations were ineffectual, for what of understanding the true Principles.

When you find on Trial, that opening the Door or a Window, enables the Chimney to carry up all the Smoke, you may be sure that want of Air from without, was the Cause of its Smoking. I say from without, to guard you against a common Mistake of those who may tell you, the Room is large, contains abundance of Air, sufficient to supply any Chimney, and therefore it cannot be that Chimney wants Air. These Reasoners are Ignorant, that the largeness of a Room, if tight, is in this case of small Importance, since it cannot part with a Chimney-full of its Air without occasioning so much Vacuum, which it requires a great Force to effect, and could not be borne if effected.

It appears plainly, then, that some of the outward Air must be admitted, the Question will be how much is absolutely necessary; for you would avoid admitting more, as being contrary to one of your Intentions in having a Fire, viz. that of warming your Room. To discover this Quantity, shut the Door gradually while a middling Fire is burning, till you find that before it is quite shut the Smoke beings to come out into the Room, then open it a little till you perceive the Smoke comes out no longer. There hold the Door, and observe the width of the open Crevice between the Edge of the Door, and the Rabbit it should shut into. Suppose the Distance to be half an Inch, and the Door 8 feet high, you find thence, that your Room requires an Entrance for Air equal in Area to 96 half Inches, or 48 square Inches, or a Passage of 6 Inches by 8. This however is a large Supposition, there being few Chimneys, that having a moderate Opening, and a tolerable Height of Funnel, will not be satisfied with such a Crevice of a ¼ of an Inch; and I have found a Square of 6 by 6, or 36 square Inches to be a pretty good Medium, that will serve for most Chimneys. High Funnels with small and low Openings, may indeed be supply’d thro’ a less Space, because for Reasons that will appear hereafter the Force of Levity, if one may so speak, being greater in such Funnels, the cool Air centers the Room with greater Velocity, and consequently more enters in the same time. This However has its Limits, for Experience shows that no increas’d Velocity so occasion’d, has made the Admission of Air thro’ the Key-hole, equal in Quantity to that thro’ an open Door, tho’ thro’ the Door the Current moves slowly, and thro’ the Key-hole with great Rapidity.

It remains then to be considered how and where this necessary Quantity of Air from without is to be admitted so as to be least inconvenient. For if at the Door, left so much open, the Air thence proceeds directly to the Chimney, and in its way comes cold to your Back and Heels as you sit before your Fire. If you keep the Door shut, and raise a little the Sash of your Window, you feel the same Inconvenience. Various have been the Contrivances to avoid this, such as bringing in fresh Air thro’ Pipes in the James of the Chimney, which pointing upwards should blow the Smoke up the Funnel: Opening Passages into the Funnel above, to let in Air for the same purpose. But these produce an Effect contrary to that intended: For as it is the constant Current of Air passing from the Room thro’ the Opening of the Chimney into the Funnel which presents the Smoke’s coming out into the Room, if you supply the Funnel by other Means or in other ways with the Air it wants, and especially if that Air be cold, you diminish the Force of that Current, and the Smoke in its Efforts to enter the Room finds less Resistance.

The wanted Air must then indispensably be admitted into the Room, to supply what goes off thro’ the Opening of the Chimney. M. Gauger, a very ingenious and intelligent French Writer on the Subject, proposes with Judgment to admit it above the Opening of the Chimney, and to prevent any Inconvenience from its Coldness, he directs its being made to pass in its Entrance thro’ winding Cavities made behind the Iron Back and Sides of the Fireplace; and under the Iron Hearth Plate; in which Cavities it will be warmed, and even heated, so as to contribute much, instead of cooling, to the Warming of the Room. This Invention is excellent in itself, and may be us’d with Advantage in building new Houses; because the Chimneys may then be so disposed, as to admit conveniently the cold Air to enter such Passages: But in Houses built without such Views, the Chimneys are often so situated, as not to afford that Convenience without great and expensive Alteration. Easy and cheap Methods, tho’ not quite so perfect in themselves, are of more general Utility, and such are the following.

In all Rooms where there is a Fire, the Body of Air warmed and rarefied before the Chimney is continually changing Place, and making room for other Air that is to be warmed in its turn. Part of it enters and goes up the Chimney, and the rest rises and takes Place near the Cieling. If the Room be lofty, that warm Air remains above our Heads as long as it continues warm, and we are little benefited by it, because it does not descend till it is cooler. Few can imagine the Difference of Climate between the upper and lower Parts of such a Room, who have not try’d it by the Thermometer, or by going up a Ladder till their Heads are near the Cieling. It is then among this warm Air that the wanted Quantity of outward Air is best admitted, with which being mix’d, its Coldness is abated, and its Inconveneince diminished so as to be some scarce observable. This may be easily done, by drawing down about an Inch the upper Sash of a Window; or, if not moveable, by cutting such a Crevice tho’ its Frame; in both which Cases, it will be well to place a think Shelf of the length, to conveal the Opening, and sloping upward to direct the entring Air horizontally along and under the Cieling. In some Houses the Air may be admitted by such a Crevice made in the Wainscot, Cornish, or Plastering, near the Cieling, and over the Opening of the Chimney. This, if practicable, is to be chose, because the entring cold Air will there meet with the warmest rising Air from before the Fire, and be soonest tempered by the Mixture. The same kind of Shelf should also be placed here. Another way, and not a very difficult one, is to take out an upper Pane of Glass in one of your Sashes, set it in a Tin Frame, giving it two springing angular Sides, and then replacing it, with Hinges below on which it may be turned to open more or less above. It will then heave the Appearance of an internal Skylight. By drawing this Pane in, more or less, you may admit what Air you find necessary. Its Position will naturally throw that Air up and along the Cieling. This is what is called in France a Was ist Das? As this is a German Question, the Invention is probably of that Nation, and takes its Name from the frequent asking of that Question when it first appeared. In England some have of late Years cut a round Hole about 5 Inches diamenter in a Pane of the Sash and plac’d against it a circular Plate of Tin hung on an Axis, and cut into Vanes, which being separately bent a little obliquely, are acted upon by the entring Air so as to force the Plate continually round like the Vanes of a Windmill. This admits the outward Air, and by the continual Whirling of the Vanes, does in some degree disperse it. The Noise only is a little inconvenient.

2. A second Cause of the Smoking of Chimneys, is, Their Openings in the Room being too large; that is two wide, too high or both. Architects in general have no other Ideas of Proportion in the Opening of a Chimney, than what relate to Symmetry and Beauty, respecting the Dimensions of the Room; while its true Proportion, respecting its Function and Utility depends on quite other Principles; and they might as properly proportion the Step in a Staircase to the Height of the Story, instead of the natural Elevation of Mens Legs in mounting. The Proportion then to be regarded, is what relates to the Height of the Funnel. For as the Funnels in the different Stories of a House are necessarily of different Heights or Lengths, that from the lowest Floor being the highest or longest, and those of the other Floors shorter and shorter, till we come to those in the Garrets, which are of course the shortest; and the Force of Draft being, as already said, in proportion to the height of Funnel filled with rarefied Air, and a Current of Air from the Room into the Chimney sufficient to fill the Opening being necessary to oppose and prevent the Smoke’s coming out into the Room; it follows that the Openings of the longest Funnels may be larger, and that those of the shorter Funnels should be smaller. For if there be a large Opening to a Chimney that does not draw strongly, the Funnel may happen to be furnish’d with the Air it demands by a partial Current entring on one side of the Opening, and leaving the other side free of any Opposing Current, may permit the Smoke to issue there into the Room. Much too of the Force of Draft in a Funnel depends on the degree of Rarefaction in the Air it contains, and that depends on the nearness to the Fire of its Passage in entring the Funnel. If it can enter far from the Fire on each Side, or far above the Fire, in a wise or high Opening, it receives little heat in passing by the Fire, and the Contents of the Funnel is by that means less different in Levity from the surrounding Atmosphere, and its Force in drawing consequently weaker. Hence if too large an Opening be given to Chimneys in upper Rooms, those Rooms will be smoky: On the other Hand if too small Openings be given to Chimneys in the lower Rooms, the entrying Air operating too directly and violently on the Fire, and afterwards strengthening the draft as it ascens the Funnel, will consume the Fuel too rapidly.

As different Circumstances frequently mix themselves in these Matters, it is difficult to give precise Dimensions for the Openings of all Chimneys. Our Fathers made them generally much too large. We have lessen’d them, but they are often still of greater Dimension than they should be, the human Eye not being easily reconcil’d to sudden and great Changes. If you suspect that your Chimney smokes from the too great Dimension of its Opening, contract it by placing Boards so as to lower and narrow it gradually, till you find the Smoke no longer issues into the Room. The Proportion so found will be that which is proper for that Chimney, and you may employ the Brick layer or Mason to reduce it accordingly. However, as in building new Houses, something must be sometimes hazarded, I would make the Openings in my lower Rooms about 30 Inches square, and 18 deep; and those in the upper only 18 Inches square and not quite so deep; the intermediate ones diminishing in proportion as the Height of Funnel diminish’d. In the larger Openings, Billets of two feet long, or half the common length of Cordwood, may be burnt conveniently; and for the smaller, such Wood may be sawed into Thirds. Where Coals are the Fuel; the Grates will be proportion’d to the Openings. The same Depth is nearly necessary to all, the Funnels being all made of a Size proper to admit a Chimney-sweeper. If in large and elegant Rooms Custom or Fancy should require the Appearance of a larger Chimney, it may be from’d of expensive marginal Decorations, in Marble. &c. In time perhaps that which is fittest in the nature of things, may come to be thought handsomest. But at present when Men and Women in different Countries show themselves dissatisfied with the Forms God has given to their Heads, Waists and Feet, and pretend to shape them more perfectly, it is hardly to be expected that they will be content always with the best Form of a Chimney. And there are some I know so bigotted to the Fancy of a large noble Opening, that rather than change it, they would submit to have damaged Furniture, sore Eyes, and Skins almost smok’d to Bacon.

3. Another Cause of smoky Chimneys is too short a Funnel. This happens necessarily in some Cases, as where a Chimney is required in a low Building. For if the Funnel be rais’d high above the Roof, in order to strengthen its Draft, it is then in danger of being blown down, and crushing the Roof in its Fall.

Contract the Opening of the Chimney, so as to oblige all the entring Air to pass thro’ or very near the Fire, whereby it will be more heated and rarefied, the Funnel itself be more warmed, and its Contents have more of what may be called the Force of Levity, so as to rise strongly, and maintain a good Draft at the Opening.

Or you may in some Cases to Advantage build additional Stories over the low Building, which will support a high Funnel.

If the low Building be us’d as a Kitchen, and a Contraction of the Opening therefore inconvenient, a large one being necessary, at least when there are great Dinners, for the free Management of so many Cooking Utensils; in such Case I would advise the Building of two more Funnels joining to the first, and having three moderate Openings, one to each Funnel, instead of one large One. When there is occasion to use but one, the other two may be kept shut by Sliding Plates, hereafter to be described; and two or all of them may be used together when wanted. This will indeed be an Expence, but not a useless one, since your Cooks will work with more comfort, see better than in a smoky Kitchen what they are about, your Victuals will be cleaner drest, and not taste of Smoke as is often the Case; and it render the Effect more certain, a Stack of three Funnels may be safely built higher above the Roof than a single Funnel.

The Case of too short a Funnel is more general than would be imagin’d, and often found when one would not expect it. For it is not uncommon in ill-contriv’d Buildings instead of having a Funnel for each Room or Fireplace, to bend and turn the Funnel of an upper Room so as to make it enter the Side of another Funnel that comes from below. By this means the upper-room Funnel is made short of course, since its Length can only be reckon’d from the Place where it enters the lower-room Funnel; and that Funnel is also shorten’d by all the Distance between the Entrance of the second Funnel and the Top of the Stack: For all that Part being readily supply’d with Air thro’ the second Funnel, adds no Strength to the Draft, especially as that Air is cold when there is no Fire in the second Chimney. The only easy Remedy here is, to keep the Opening shut of that Funnel in which there is no Fire.

4. Another very common Cause of the Smoking of Chimneys, is, their Overpowering one another. For instance, if there be two Chimneys in one large Room, and you make Fires in both of them, the Doors and Windows close shut, you will find that the greater and stronger Fire shall overpower the weaker, and draw Air down its Funnel to supply its own Demand; which Air descending in the weaker Funnel will drive down its Smoke and force it into the Room. If instead of being in one Room, the two Chimneys are in two different Rooms, communicating by a Door, the Case is the same whenever that Door is open. In a very tight House, I have known a Kitchen Chimney on the lowest Floor, when it had a great Fire in it, overpowering any other Chimney in the House, and draw Air and Smoke into its Room, as often as the Door was open’d communicating with the Stair-Case.

Take Care that every Room have the Means of supplying itself from without with the Air its Chimney may require, so that no one of them may be oblig’d to borrow from another, nor under the Necessity of lending. A Variety of these Means have been already describ’d.

5. Another Cause of Smoking is, when the Tops of Chimneys are commanded by high Buildings, or by a Hill, so that the Wind blowing over such Eminences falls like Water over a Dam, sometimes almost perpendicularly on the Tops of the Chimneys that lie in its way, and beats down the Smoke contain’d in them.

That commonly apply’d to this Case, is a Turn Cap made of Tin or Plate Iron, covering the Chimney above and on three Sides, open on one Side, turning on a Spindle, and which being guided or governed by a Vane, always presents its back to the Current. This I believe may be generally effectual, tho’ not certain, as there may be Cases in which it will not succeed. Raising your Funnels if practicable, so as their Tops may be higher, or at least equal with the commanding Eminence, is more to be depended on. But the turning Cap being easier and cheaper should first be try’d. If oblig’d to build in such a Situation, I would chuse, to place my Doors on the Side next the Hill, and the Backs of my Chimneys on the farthest Side, for then the Column of Air falling over the Eminence, and of course pressing on that below, and forcing it to enter the Doors, or Was ist Dasses on that Side, would tend to ballance the Pressure down the Chimneys, and leave the Funnels more free in the Exercise of their Functions.

6. There is another Case of Command, the Reverse of that last mentioned. It is where the commanding Eminence is farther from the Wind than the Chimney commanded. To explain this a Figure may be necessary. Suppose then a Building whose Side A happens to be expos’d to the Wind, and forms a kind of Dam against its Progress. The Air obstructed by this Dam will like Water press and search for Passages thro’ it; and finding the Top of the Chimney B, below the Top of the Dam, it will force itself down that Funnel in order to get through by some Door or Window open on the other Side of the Building[, and the rather as the Air on that other Side is made rarer by the Force of the Current.] And if there be a Fire in such Chimney, its Smoke is of course beat down, and fills the Room.

I know of but one, which is to raise such Funnel higher than the Roof, supporting it if necessary by Iron Bars. For a Turn-cap in this Case has no Effect, the damm’d-up Air pressing down thro’ it in whatever Position the Wind may have plac’d its Opening.

I know a City in which many Houses are render’d smoky by this Operation. For their Kitchens being built behind, and connected by a Passage with the Houses, and the Tops of the Kitchen Chimneys lower than the Top of the Houses, the whole Side of a Street when the Wind blows against its back, forms such a Dam as above describ’d; and the Wind so obstructed forces down those Kitchen Chimneys, (especially when they have but weak Fires in them) to pass thro’ the Passage and House, into the Street. Kitchen Chimneys so form’d and situated, have another Inconvenience. In Summer if you open your Upper Room Windows for Air, a light Breeze blowing over your Kitchen Chimney towards the House, tho’ not strong enough to force down its Smoke as aforesaid, is sufficient to waft it into your Windows, and fill the Room with it, which besides the Disagreableness, damages your Furniture.

7. Chimneys, otherwise drawing well, are sometimes made to smoke, by the improper and inconvenient Situation of a Door. When the Door and Chimney are on the same Side of the Room as in the Figure, if the Door being in the Corner is made to open against the Wall, which is common as being there, when open, more out of the Way; it follows, that when the Door is only open’d in Part, a Current of Air rushing in passes along the Wall into and across the Opening of the Chimney B, and flirts some of the Smoke out into the Room. This happens more certainly when the Door is shutting for then the Force of the Current is augmented; and becomes very inconvenient to those who, warming themselves by the Fire, happen to sit in its way.

Are obvious and easy. Either put an intervening Skreen, from the Wall round great Part of the Fire Place; Or, which is perhaps preferable, shift the Hinges of your Door, so as it may open the other way, and when open throw the Air along the other Wall.

8. A Room that has no Fire in its Chimney, is sometimes filled with Smoke which is receiv’d at the Top of its Funnel and descends into the Room. In a former Paper I have already explained the descending Currents of Air in cold Funnels; it may not be amiss however to repeat here, that Funnels without Fires have an Effect according to their Degrees of Coldness or Warmth, on the Air that happens to be contained in them. The surrounding Atmosphere is frequently changing its Temperature; but Stacks of Funnels cover’d from Winds and Sun by the House that contains them, retain a more equal Temperature. If after a warm Season, the outward Air suddenly grows cold, the empty warm Funnels begin to draw strongly upward; that is, they rarefy the Air contain’d in them, which of course rises, cooler 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 aClock 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 contantly 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.

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 back 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 nost frequent where the Funnel is short, and the Opening turn’d from the Wind. It is the more grevious, when it 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 to rise the Air that is over it. In a Time of Calm or of little Wind, this is done visibly, for we see the Smoke 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, 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.

In some Places, particularly in Venice, the Custom is, to open or widen the Top of the Flue rounding in the true Form of a Funnel, thus, 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 practic’d, the Tops of the Tlues being narrow’d inwards, so as to form a Slit for the Issue of the Smoke, long as the Breadth of the Funnel, and only 4 Inches wide. This seems to have been contriv’d on a Supposition that the Entry of the Wind would thereby be obstructed, and perhaps it might have been imagined, that the whole Force of the rising warm Air being condens’d, as it were, in the narrow Opening, would thereby be strengthen’d, so as to overcome the Resistance of the Wind. This however did not always succeed; for when the Wind was at North West and blew fresh, the Smoke was forc’d down by Fits into the Room I commonly sat in, so as to oblige me to shift the Fire into another. The Position of the Slit of this Funnel was indeed N.E. and S. West. Perhaps if it had lain across the Wind, the Effect might have been different. But on this I can give no Certainty. It seems a Matter proper to be referr’d to Experiment. Possibly a Furn-cap might have been serviceable; but it was not tried.

Chimneys have not been long in Use in England. I remember to have formerly read in some very old Book which remark’d the then modern Improvements of Living, and mention’d among others the Convenience of Chimneys. “Our Fore Fathers said the Author, had no Chimneys. There was in each Dwelling House only one Place for a Fire, and the Smoke went out thro’ a Hole in the Roof; but now there is scarce a Gentleman’s House in England that has not at least one Chimney in it.” When there was but one Chimney, its Top might then be open’d as a Funnel, and perhaps borrowing the Form from the Venetians, it was then the Flue of a Chimney got that Name. Such is now the Growth of Luxury, that in both England and France we must have a Chimney for every Room, and in some Houses, every Possessor of a Chamber, and almost every Servant, will have a Fire; So that the Flues being necessarily built in Stacks, the opening of each as a Funnel is impracticable. This Change of Manners soon consum’d the Fire-wood of England, and will soon render Fuel extreamly scarce and dear in France, if the Use of Coals be not introduc’d in the latter Kingdom as it has been in the former; where it at first met with Opposition; for there is extant in the Records of one of Queen Elizebeth’s Parliaments, a Motion made by a Member, reciting, “that many Dyers, Brewers, Smiths and other Artificers of London, had of late taken to the Use of Pitcoal for their Fires, instead of Wood, which fill’d the Air with noxious Vapours and Smoke, very prejudicial to the Health, particularly of Persons coming out of the Country; and therefore moving that a Law might pass to prohibit the Use of such Fuel, (at least during the Session of Parliament) by those Artificers.” It seems it was not then commonly us’d in private Houses. It’s suppos’d UnWholesomeness was an Objection. Luckily the Inhabitants of London, have got over that Objection, and now think it rather contributes to render their Air salubrious, as they have had no general pestilential Disorder since the general Use of Coals, when before it such were frequent. Paris still burns Wood at an enormous Expence continually augmenting, the Inhabitants having still that Prejudice to overcome. In Germany you are happy in the Use of Stoves which saves Fuel wonderfully: Your People are very ingenious in the Management of Fire; but they may still learn something in that Art from the Chinese, whose Country being greatly populous and fully cultivated, has little room left for the Growth of Wood, and having not much other Fuel that is good, have been forc’d upon many Inventions during a Course of Ages, for making a little Fire go as far as possible.

I have thus gone thro’ all the common Causes of the Smoking of Chimneys that I can at present recollect as having fallen under my Observation, communicating the Remedies that I have known successfully used for the different Cases, together with the Principles on which both the Disease and the Remedy depend, and confessing my Ignorance wherever I have been sensible of it. You will do well, if you publish, as you propose, this Letter, to add in Notes or as you please, such Observations as may have occur’d to your attentive Mind; and if other Philosophers will do the same, this Part of Science, tho’ humble yet of great Utility, may in time be perfected. For many Years past, I have rarely met with a Case of a Smoky Chimney, which has not been soluable on these Principles, and cur’d by these Remedies where People have been willing to apply them; which is indeed not always the Case; for many have Prejudices in favour of the Nostrum of pretending Chimney-Doctors and Fumists, and some have Conceits and Fancies of their own, which they rather chuse to try, than to lengthen a Funnel, alter the Size of an Opening, or admit Air into a Room however necessary; for some are as much afraid of fresh Air as Persons in the Hydrophobia are of fresh Water. I myself had formerly this Prejudice, this Aerophobia, as I now account it, and reading the suppos’d dangerous Effects of cool Air, I consider’d it as an Enemy, and clos’d with extreme Care every Crevice in the Rooms I inhabited. Experience has convinced me of my Error. I now look upon fresh Air as a Friend. I even sleep with an open Window. I am persuaded that no common Air from without, is so unwholesome as the Air within a close Room, that has been often breath’d and not changed. Moist Air, too, which formerly I thought pernicious, gives me now no Apprehensions: For considering that no Dampness of Air apply’d to the Outside of my Skin, can be equal to what is apply’d to and touches it within, my whole Body being full of Moisture; and finding that I can lie two hours in a Bath twice a Week, cover’d with Water, which certainly is much damper than any Air can be, and this for Years together, without catching Cold, or being in any other manner disorder’d by it, I no longer dread mere Moisture, either in Air or Sheets or Shirts: And I find it of Importance to the Happiness of Life, the being freed from vain Terrors, especially of Objects, that we are every day exposed inevitably to meet with. You Physicians have of late happily discover’d, after a contrary Opinion had prevail’d some Ages, that fresh and cool Air does good to Persons in the Small Pox and other Fevers. It is to be hop’d that in another Century or two we may all find out, that it is not bad even for People in Health. And as to moist Air, here I am at this present Writing in a Ship with above 40 Persons, who have had no other but moist Air to breathe for 6 Weeks past, every thing we thouch is damp, and nothing dries; yet we are all as healthy as we should be on the Mountains of Switzerland, whose Inhabitants are not more so than those of Bermuda or St. Helena, Islands on whose Rocks the Waves are dash’d into Millions of Particules, which fill the Air with Damp, but produce no Diseases, the Moisture being pure, unmix’d with the poisonous Vapours arising from putrid Marshes, and stagnant Pools, in which many Insects die and corrupt the Waters. These Places only, in my Opinion, (which I submit to yours) afford unwholsome Air; and that it is not the mere Water contain’d in damp Air, but the volatile Particles of corrupted animal Matter mix’d with that Water, which renders such Air pernicious to those who breathe it. And I imagine it a Cause of the same kind that renders the Air in close Rooms, where the perspirable Matter is breath’d over and over again by a number of assembled People, so hurtful to Health. After being in such a Situation, many find themselves affected by that Febricula, which the English alone call a cold, and perhaps from the Name imagine that they caught the Malady by going out of the Room when it was infact by being in it.

You begin to think that I wander from my Subject and go out of my Depth. So I return again to my Chimneys.

We have of late many Lecturers in Experimental Philosophy. I have wish’d that some of them would study this Branch of that Science, and give Experiments in it as a Part of their Lectures. The addition to their present Apparatus need not be very expensive. A number of little Representations of Rooms compos’d each of 5 Panes of Sash Glass, fram’d in Wood at the Corners, with proportionable Doors, and moveable Glass Chimneys, with Openings of different Sizes, and different Lengths of Funnel; and some of the Rooms so contriv’d as to communicate on occasion with others, so as to form different Combinations, and exemplify different Cases; with quantities of gree Wax Taper cut into Pieces of an Inch and half, 16 of which stuck together in a Square, and lit, would make a strong Fire for a little Glass Chimney, and blown out would continue to burn and give Smoke as long a desired. With such an Apparatus all the Operations of Smoke and rarifi’d Air in Rooms and Chimneys might be seen thro’ their transparent Sides; and the Effect of Winds on Chimneys, commanded or otherwise, might be shown by letting the entring Air blow upon them thro’ an opened Window of the Lecturer’s Chamber, where it would be constant while he kept a good Fire in his Chimney. By the help of such Lectures our Fumists would be some better instructed. At present they have generally but one Remedy, which perhaps they have know effectual in some one Case of Smoky Chimneys, and they apply that indiscriminately to all the other Cases, without success, but not without Expence to their Employers.

With all the Science however, that a Man shall suppose himself possess’d of in this Article, he may sometimes meet with Cases that shall puzzle him. I once lodg’d in a House at London, which, in a little Room had a single Chimney and Funnel. The Opening was very small, yet it did not keep in the Smoke, and all Attempts to have a Fire in this Room were fruitless. I could not imagine the Reason, till at length observing that the Chamber over it, which had no Fireplace in it, was always filled with Smoke when a Fire was kindled below, and that the Smoke came thro’ the Cracks and Crevices of the Wainscot, I had the Wainscot taken down, and discover’d that the Funnel which went up behind it, had a Crack many feet in Length, and wide enough to admit my Arm, a Breach very dangerous with regard to Fire, and occasion’d probably by an apparent irregular Settling of one Side of the House. The Air entring this Breach freely, destroy’d the drawing Force of the Funnel. The Remedy would have been filling up the Breach or rather rebuilding the Funnel. But the Landlord rather chose to stop up the Chimney.

Another puzzling Case I met with at a Friend’s Country House near London. His best Room had a Chimney in which he told me he never could have a Fire, for all the Smoke came out into the Room. I flatter’d my self I could easily find the Cause and prescribe the Cure. I had a Fire made there, and found it as he said. I opened the Door, perceiv’d it was not want of Air. I made a temporary Contraction of the Opening of the Chimney, and found that it was not its being too large that caus’d the Smoke to issue: I went out and look’d up at the Top of the Chimney: Its Funnel was join’d in the same Stack with others, some of them shorter, that drew very well, and I saw nothing to prevent its doing the same. In fine after every other Examination I could think of, I was oblig’d to own the Insufficiency of my Skill. But my Friend afterwards discover’d the Cause himself. He got to the Top of the Funnel by a Ladder, and looking down found it filled with Twiggs and Straw cemented by Earth, and lin’d with Feathers. It seems the House after being built, had stood empty some Years before he occupy’d it; and he concluded that some large Birds had taken the Advantage of its retired Situation to make their Nest there. The Rubbish, considerable in Quantity, being removed, and the Funnel cleared, the Chimney drew well, and gave Satisfaction.

In general Smoke is a very tractable Thing, easily governed and directed when one knows the Principles, and is well informed of the Circumstances. You know I made it descend in my Pennsylvania Stove. I formerly had a more simple Construction in which the same Effect was produc’d, but visable to the Eye. It was compos’d of two Plates A.B. and C.D. plac’d as in this Figure The lower Plate AB rested with its Edge in the Angle made by the Hearth with the Back of the Chimney. The upper Plate was fix’d to the Breast, and laps over the lower about 6 Inches leaving a Space of 4 Inches wide and the length of the Plates (near 2 feet) between them. Every other Passage of Air into the Funnel was well stopt. When therefore a Fire was made at E, for the first time with Charcoal till the Air in the Funnel was a little heated thro’ the Plates, and then Wood laid on, the Smoke would rise to A. turn over the Edge of that Plate, descend to D, then turn under the Edge of the upper Plate, and go up the Chimney. It was pretty to see, but of no great Use. Placing therefore the under Plate in a higher Situation, I remov’d the Upper Plate C.D. and plac’d it perpendicularly, thus, so that the upper Edge of the lower Plate A.B. came within about 3 Inches of it, and might be push’d farther from it, or suffer’d to come nearer to it by a moveable Wedge between them. The Flame then ascending from the Fire at E was carried to strike the upper Plate, made it very hot, and its Heat rose and spread with the rarefied Air into the Room.

I believe you have seen in use with me, the Contrivance of a Sliding-Plate over the Fire, seemingly plac’d to oppose the rising of the Smoke, leaving but a small Passage for it, between the Edge of the Plate and the Back of the Chimney. It is particularly describ’d and its Uses explain’d in my former printed Letters, and I mention it here only as another Instance of the Tractability of Smoke.

What is called the Staffordshire Chimney affords an Example of the same kind. The Opening of the Chimney is brick’d up, even with the Fore Edge of its Jambs, leaving open only a Passage over the Grate of the same width and perhaps 8 Inches high. The Grate consists of semicircular Bars, the upper Bar of the greatest Diameter, the others under it smaller and smaller, so that it has the Appearance of half a round Basket. It is, with the Coals it contains, wholly without the Wall that shuts up the Chimney, yet the Smoke bends and enters the Passage above it, the Draft being strong, because no Air can enter that is not oblig’d to pass near or through the Fire, so that all that the Funnel is fill’d with is much heated and of course much rarified.

Much more of the Prosperity of a Winter Country depends on the Plenty and Cheapness of Fuel, than is general imagined. In Travelling I have observed, that in those Parts where the Inhabitants can have neither Wood nor Coal nor Turft but at excessive Prices, the Working People live in miserable Hovels, are ragged, and have nothing comfortable about them. But where Fewel is cheap, (or where they have the Art of managing it to Advantage) they are well furnish’d with Necessaries, and have decent Habitations. The obvious Reason is, that the Working Hours of such People are the profitable Hours, and they who cannot afford sufficient Fuel have fewer such Hours in the 24 than those who have it cheap and plenty. For much of the domestic Work of poor Women, such as Spinning, Sewing, Knitting, and of the Men in those Manufactures that require little bodily Exercise, cannot well be perform’d where the Fingers are numb’d with Cold; Those People therefore in cold Weather are induc’d to go to bed sooner and lie longer in a Morning, than they would do if they could have good Fires or warm Stoves to sit by; and their Hours of Work are not sufficient to produce the Means of comfortable Subsistence. Those publick Works, therefore, such as Roads, Canals, &c. by which Fuel may be brought cheap into such Countries from distant Places, are of great Utility; and those who promote them may be reckoned among the Benefactors of Mankind.

I have great Pleasure in having thus comply’d with your Request and in the Reflection that the Friendship you honour me with, and in which I have ever been so happy, has continued so many Years without the smallest Interruption. Our Distance from each other is now augmented, and Nature must soon put an End to the possibility of my continuing our Correspondence: But Consciousness and Memory remain in a future State, my Esteem and Respect for you, my dear Friend, will be everlasting.