a dense elastic vapour, arising from burning bodies. As this vapour is extremely disagreeable to the senses, and often prejudicial to the health, mankind have fallen upon several contrivances to enjoy the benefit of fire, without being annoyed by smoke. The most universal of these contrivances is a tube leading from the chamber in which the fire is kindled to the top of the building, through which the smoke ascends, and is dispersed into the atmosphere. These tubes are called chimneys; which, when constructed in a proper manner, carry off the smoke entirely; but, when improperly constructed, they carry off the smoke imperfectly, to the great annoyance of the inhabitants. As our masters at present seem to have a very imperfect knowledge of the manner in which chimneys ought to be built, we can hardly perform a more acceptable service to the public than to point out the manner in which they ought to be constructed, so as to carry off the smoke entirely; as well as to explain the causes from which the defects so often complained of generally proceed, and the method of removing them.

Those who would be acquainted with this subject, should begin by considering on what principle smoke ascends in any chimney. At first many are apt to think that smoke is in its nature, and of itself, specifically lighter than air, and rises in it for the same reason that cork rises in water. These see no cause why smoke should not rise in the chimney though 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 lighted a pipe of tobacco, plunge the stem to the bottom of a decanter half filled with cold water; then putting a rag over the bowl, blow through it, and make the smoke descend in the stem of the pipe, from the end of which it will rise in bubbles through the water; and being thus cooled, will not afterwards rise to go out through 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 attached to or acted upon by air that is heated, and thereby rarefied and rendered specifically lighter than the air in its neighbourhood.

Smoke being rarely seen but in company with heated air, and its upward motion being visible, though that of the rarefied air that drives it is not so, has naturally given rise to the error. It is now well known that air is a fluid which has weight as well as others, though about 800 times lighter than water; that heat makes the particles 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 weights of cold air which may surround it, and in that case must rise, being forced upwards by such colder and heavier air, which presses to get under it and take its place. That air is so rarefied 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 on legs so that it need not be handled, for the hands might warm it. At the end of a quill fasten five or six inches of the finest light filament 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 through 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 straight, narrow below and widening upwards, Smoke. wards, 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 entering below and passing up through it till discharged 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 thereby 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 entering the tube below, is heated and rarefied by passing near and over that knob.

That this motion is produced 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 contained in a glass jar a foot deep, through 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 a very little above. As long as you keep the bottom stopped the fluids remain at rest; but the moment it is unstoppt, 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 filled 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 prehends to enter the funnel below, and so drives up before it the smoke and warm air it meets with in its passage.

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 may be reduced 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 hands, the joints of the boards of the flooring, and of the panels of wainscotting, are all true and tight; the more so as the walls, perhaps not yet thoroughly dry, preserve a dampness in the air of the room which keeps the woodwork swelled and close. The doors and the fastenings too, being worked 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 from coming out into the room. If the motion upwards of the air in a chimney that is freely supplied 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 supplied from the room below, it will appear absolutely impossible that this operation should go 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 impossibilities. Yet under this situation it is not uncommon to see 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 will carry off the smoke unless a door or window be left open. Much expense has also been made to alter and amend new chimneys which had really no fault: in one house particularly which Dr Franklin knew that belonged to a nobleman in Westminster, that expense amounted to no less than 300l. after his house had been, as he thought, finished and all charges paid. And after all, several of the alterations were ineffectual, for want of understanding the true principles.

Remedies. 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 is the cause of its smoking. "I say from without," (adds Dr Franklin), "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 the 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 chimneyful of its air without occasioning so much vacuum; which it requires a great force to effect, and could not be borne if effected."

It appearing 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 begins 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 rabbet it should shut into. Suppose the distance to be half an inch, and the door eight 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 quarter of an inch: Dr Franklin found a square of 6 by 6, or 36 square inches, to be pretty good medium that will serve for most chimneys. High funnels with small and low openings may indeed be supplied through 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 enters the room with greater velocity, and consequently more enters in the same time. This, however, has its limits; for experience shows, that no increased velocity so occasioned has made the admission of air through the key-hole equal in quantity to that through an open door, though through the door the current moves slowly, and through 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 too 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 through pipes in the jams 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 through the opening of the chimney into the funnel which prevents the smoke from coming out into the room, if you supply the funnel by other means or in other ways with the air which 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 through 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 inconvenience from its coldness, he directs that it may be so made, that it shall pass in its entrance through winding cavities made behind the iron back and sides of the fire-place, 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 used 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 alterations. Easy and cheap methods, though 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 ceiling. 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 tried it by the thermometer, or by going up a ladder till their heads are near the ceiling. It is then among this warm air that the wanted quantity of outward air is best admitted, with which being mixed, its coldness is abated, and its inconvenience diminished so as to become scarcely 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 through its frame; in both which cases it will be well to place a thin shelf of the length to conceal the opening, and sloping upwards, to direct the entering air horizontally along and under the ceiling. In some houses the air may be admitted by such a crevice made in the wainscot, cornice, or plaitering, near the ceiling and over the opening of the chimney. This, if practicable, is to be chosen, because the entering cold air will there meet with the warmest rising air from before the fire, and be soonest tempered by the mixture. The fame kind of shelf should also be placed here. Another Fig. 2. 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 have the appearance of an internal sky-light. 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 ceiling. 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 five inches diameter in a pane of the sash, and placed 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 entering 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, too 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 men's 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 from 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 Smoke may happen to be furnished with the air which it demands by a partial current entering 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 entering the funnel. If it can enter far from the fire on each side, or far above the fire, in a wide or high opening, it receives little heat in passing by the fire, and the contents of the funnel are by those 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 entering air operating too directly and violently on the fire, and afterwards strengthening the draft as it ascends the funnel, will consume the fuel too rapidly.

Remedy. 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 lessened them; but they are often still of greater dimensions than they should be, the human eye not being easily reconciled to sudden and great changes. If you suspect that your chimney smokes from the too great dimension of its opening, contract it by placing moveable 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 bricklayer or mason to reduce it accordingly. However, as in building new houses something must be sometimes hazarded, Dr Franklin proposes to make the openings in the 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 the funnel is diminished. 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 fowed into thirds. Where coals are the fuel, the grates will be proportioned 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 formed of expensive marginal decorations, in marble, &c. But in time perhaps, that which is fitted in the nature of things may come to be thought handsome.

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 raised 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.

Remedies. Contract the opening of the chimney, so as to oblige all the entering air to pass through 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 flues over the low building, which will support a high funnel.

If the low building be used 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 the best expedient perhaps would be to build 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 expense, but not an useless one, since your cooks will work with more comfort, see better than in a smoky kitchen what they are about, your viands will be cleaner dressed and not taste of smoke, as is often the case; and to 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 imagined, and often found where one would not expect it. For it is not uncommon, in ill-contrived buildings, instead of having a funnel for each room or fire place, 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 these means the upper room funnel is made short of course, since its length can only be reckoned from the place where it enters the lower room funnel; and that funnel is also shortened by all the distance between the entrance of the second funnel and the top of the stack: for all that part being readily supplied with air through 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 of that funnel shut 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 that, you will find that the greater and stronger fire shall overpower the weaker, from the funnel of which it will draw air down 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, a kitchen chimney on the lowest floor, when it had a great fire in it, has been known to overpower any other chimney in the house, and draw air and smoke into its room as often as the door communicating with the stair-case was opened.

Remedy. Take care that every room have the means of supplying itself from without with the air which its chimney may require, so that no one of them may be obliged to borrow from another, nor under the necessity of lending. A variety of these means have been already described.

5. Another cause of smoking is, when the tops of chimneys are commanded by higher 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 contained in them.

To illustrate this, let A (fig. 3.) represent a small building at the side of a great rock B, and the wind coming in the direction CD; when the current of air comes to the point D, being hurried forward with great velocity, it goes a little forward, but soon descends downward, and gradually is reflected more and more inward, as represented by the dotted lines EE, &c. so that, defending downwards upon the top of the chimney A, the smoke is beat back again into the apartments.

It is evident that houses situated near high hills or thick woods will be in some measure exposed to the same inconvenience; but it is likewise plain, that if a house be situated upon the slope of a hill (as at F, fig. 3.), it will not be in any danger of smoke when the wind blows towards that side of the hill upon which it is situated; for the current of air coming over the house-top in the direction GH, is immediately changed by the slope of the hill to the direction HC, which powerfully draws the smoke upward from the top of the chimney. But it is also evident, that a house in this situation will be liable to smoke when the wind blows from the hill; for the current of air coming downward in the direction CH, will beat downward on the chimney F, and prevent the smoke from ascending with freedom. The effect will be much heightened if the doors and windows are chiefly in the lowermost side of the house.

Remedy. That commonly applied in this case is a turncap 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 may be generally effectual, though 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 tried. "If obliged to build in such a situation, I would choose (says Dr Franklin) to place my doors on the side next the hill, and the backs of my chimneys on the farther 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-i/ll-dases on that side, would tend to balance the pressure down the chimneys, and leave the funnels more free in the exercise of their functions."

6. There is another case which is 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 AB happens to be exposed to the wind, and forms a kind of dam against its progress. Suppose the wind blowing in the direction FE. The air obstructed by this dam or building AB will like water press and search for passages through it; but finding none, it is beat back with violence, and spreads itself on every side, as is represented by the curved lines e, e, e, e, e, e. It will therefore force itself down the final chimney C, in order to get through by some door or window open on the other side of the building. And if there be a fire in such chimney, its smoke is of course beat down, and fills the room.

Remedy. There is but one remedy, which is to raise such a funnel higher than the roof, supporting it if necessary by iron bars. For a turncap in this case has no effect, the dammed-up air pressing down through it in whatever position the wind may have placed its opening.

Dr Franklin mentions a city in which many houses are rendered smoky by this operation. For their kitchens being built behind, and connected by a passage with the housetops, and the tops of the kitchen chimneys lower than the tops of the housetops, the whole side of a street when the wind blows against its back forms such a dam as above described; and the wind so obstructed forces down those kitchen-chimneys (especially when they have but weak fires in them) to pass through the passage and house into the street. Kitchen-chimneys so formed 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, though not strong enough to force down its smoke as aforesaid, is sufficient to waft it into your windows, and fill the rooms with it; which, besides the disagreeableness, 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, 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 opened in part, a current of air rushing in passes along the wall into and across the opening of the chimney, 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 fit in its way.

The remedies are obvious and easy. Either put an intervening screen from the wall round great part of the fireplace; 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 received at the top of its funnel, and deflected into the room. Funnels without fires have an effect according to their degree 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 covered from winds and fun 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 contained in them, which of course rises, cooler air enters below to supply its place, is refreshed 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 contained in the cool funnels being heavier descends into the room; and the warmer air which enters their tops being cooled 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 through 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 nine or ten 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 defends with the air into the chamber.

The remedy is to have a sliding plate that will shut perfectly the offending funnel. Dr Franklin has thus described it: "The opening of the chimney is contracted by brick-work faced with marble slabs to about two feet between the jams, and the breast brought down to within about three feet of the hearth. An iron frame is placed just under the breast, and extending quite to the back of the chimney, so that a plate of the same metal may slide horizontally backwards and forwards in the grooves on each side of the frame. This plate is just so large as to fill the whole space, and shut the chimney entirely when thrust quite in, which is convenient when there is no fire. Draw it out, so as to leave between its further edge and the back a space of about two inches; this space is sufficient for the smoke to pass; and so large a part of the funnel being swept by the rest of the plate, the passage of warm air out of the room, up the chimney, is obstructed and retarded; and by those means much cold air is prevented from coming in through crevices, to supply its place. This effect is made manifest three ways. 1. When the fire burns briskly in cold weather, the howling or whistling noise made by the wind, as it enters the room through the crevices, when the chimney is open as usual, ceases as soon as the plate is slid in to its proper distance. 2. Opening the door of the room about half an inch, and holding your hand against the opening, near the top of the door, you feel the cold air coming in against your hand, but weakly, if the plate be in. Let another person suddenly draw it out, so as to let the air of the room go up the chimney, with its usual freedom where chimneys are open, and you immediately feel the cold air rushing in strongly. 3. If something be set against the door, just sufficient, when the plate is in, to keep the door nearly shut, by resisting the pressure of the air that would force it open: then, when the plate is drawn out, the door will be forced open by the increased pressure of the outward cold air endeavouring to get in to supply the place of the warm air that now passes out of the room to go up the chimney. In our common open chimneys, half the fuel is wasted, and its effect lost; the air it has warmed being immediately drawn off."

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, though not defending from any commanding eminence. This case is most frequent where the funnel is short and the opening turned from the wind. It is the more grievous, when it happens to be a cold wind that produces the effect, because when you most want your fire you are sometimes obliged to extinguish it. To understand this, it may be considered 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 received 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.

Remedies. In Venice, the custom is to open or widen the top of the flue, rounding it in the true form of a funnel. In other places the contrary is practised; the tops of the flues being narrowed inwards, so as to form a slit for the issue of the smoke; long as the breadth of the funnel, and only four inches wide. This seems to have been contrived 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 condensed, as it were, in the narrow opening, would thereby be strengthened, so as to overcome the resistance of wind. This, however, did not always succeed; for when the wind was at north-east and blew fresh, the smoke was forced down by fits into the room where Dr Franklin commonly sat, so as to oblige him to shift the fire into another. The position of the slit of this funnel was indeed north-east and south-west. Perhaps if it had lain across the wind, the effect might have been different. But on this we can give no certainty. It seems a matter proper to be referred to experiment. Possibly a turnep might have been serviceable, but it was not tried.

With all the science, however, that a man shall suppose himself possessed of in this article, he may sometimes meet with cases that shall puzzle him. "I once lodged (says Dr Franklin) 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 through the cracks and crevices of the wainscot; I had the wainscot taken down, and discovered 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 occasioned probably by an apparent irregular settling of one side of the house. The air entering this breach freely, destroyed 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 flattered myself I could easily find the cause and prescribe the cure. I opened the door, and perceived 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 caused the smoke to issue. I went out and looked up at the top of the chimney: its funnel was joined 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 obliged to own the insufficiency of my skill. But my friend, who made no pretension to such kind of knowledge, afterwards discovered the cause himself. He got to the top of the funnel by a ladder, and looking down found it filled with twigs and straw cemented by earth and lined with feathers. It seems the house after being built, had stood empty some years before he occupied 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."

Chimneys whose funnels go up in the north wall of a house, and are exposed to the north winds, are not so apt to draw well as those in a south wall; because when rendered cold by those winds, they draw downwards.

Chimneys inclosed in the body of a house are better than those whose funnels are exposed in cold walls.

Chimneys in stacks are apt to draw better than separate funnels, because the funnels that have constant fires in them warm the others in some degree that have none.

SMOKE-Jack. This ingenious machine is of German origin, and Messinger, in his Collection of Mechanical Performances, says it is very ancient, being represented in a painting at Nurenbergh, which is known to be older than the year 1350.

Its construction is abundantly simple. An upright iron spindle GA (fig. 5), placed in the narrow part of the kitchen chimney, turns round on two points H and I. The upper one H passes through an iron bar, which is built in across the chimney; and the lower pivot I is of tempered steel, and is conical or pointed, resting in a conical bell metal socket fixed on another cross bar. On the upper end of the spindle is a circular fly G, consisting of 4, 6, 8, or more thin iron plates, let obliquely on the spindle like the sails of a windmill, as we shall describe more particularly by and by. Near the lower end of the spindle is a pinion A, which works in the teeth of a contrate or face wheel B, turning on a horizontal axis BC. One pivot of this axis turns in a cock fixed on the cross bar, which supports the lower end of the upright spindle HI, and the other pivot turns in a cock fixed on the side wall of the chimney; so that this axle is parallel to the front of the chimney. On the remote end of this horizontal axle there is a small pulley C, having a deep angular groove. Over this pulley there passes a chain CDE, in the lower bight of which hangs the large pulley E of the spit. This end of the spit turns loosely between the branches of the fork of the rack or raxe F, but without resting on it. This is on the top of a moveable stand, which can be shifted nearer to or farther from the fire. The other end turns in one of the notches of another rack. The number of teeth in the pinion A and wheel B, and the diameters of the pulleys C and E, are so proportioned that the fly G makes from 12 to 20 turns for one turn of the spit.

The manner of operation of this useful machine is easily understood. The air which contributes to the burning of the fuel, and passes through the midst of it, is greatly heated, and expanding prodigiously in bulk, becomes lighter than the neighbouring air, and is therefore pushed by it up the chimney. In like manner, all the air which comes near the fire is heated, expanded, becomes lighter, and is driven up the chimney. This is called the draught or suction, but would with greater propriety be termed the drift of the chimney. As the chimney gradually contracts in its dimensions, and as the same quantity of heated air passes through every section of it, it is plain that the rapidity of its ascent must be greatest in the narrowest place. There the fly G should be placed, because it will there be exposed to the strongest current. The air, striking the fly vanes obliquely, pushes them aside, and thus turns them round with a considerable force. If the joint of meat is exactly balanced on the spit, it is plain that the only resistance to the motion of the fly is what arises from the friction of the pivots of the upright spindle, the friction of the pinion and wheel, the friction of the pivots of the horizontal axis, the friction of the small end of the spit, and the friction of the chain in the top pulleys. The whole of this is but a mere trifle. But there is frequently a considerable inequality in the weight of the meat on different sides of the spit: there must therefore be a sufficient overplus of force in the impulse of the ascending air on the vanes of the fly, to overcome this want of equilibrium occasioned by the unskilfulness or negligence of the cook. There is, however, commonly enough of power when the machine is properly constructed. The utility of this machine will, we hope, procure us the indulgence of some of our readers, while we point out the circumstances on which its performance depends, and the maxims which should be followed in its construction.

The upward current of air is the moving power, and should be increased as much as possible, and applied in the most advantageous manner. Every thing will increase the current which improves the draught of the chimney, and secures it from smoking. A smoky chimney must always have a weak current. For this particular, therefore, we refer to what has been delivered in the article PNEUMATICS, No 359; and the article SMOKE.

With respect to the manner of applying this force, it is evident that the best construction of a windmill sails will be nearly the best construction for the fly. According to the usual theory of the impulse of fluids, the greatest effective impulse (that is, in the direction of the fly's motion) will be produced if the plane of the vane be inclined to the axis in an angle of 54 degrees 46 minutes. But, since we have pronounced this theory to be so very defective, we had better take a determination founded on the experiments on the impulse of fluids made by the academy of Paris. These authorise us to say, that 49\( \frac{1}{2} \) or 50 degrees will be the best angle to give the vane: but this must be understood only of that part of it which is close adjoining to the axis. The vane itself must be twisted, or weathered as the millwrights term it, and must be much more oblique at its outer extremity. The exact position cannot be determined with any precision; because this depends on the proportion proportion of the velocity of the vane to that of the current of heated air. This is subject to no rule, being changed according to the load of the jack. We imagine that an obliquity of 65 degrees for the outer ends of the vanes will be a good portion for the generality of cases. Messinger describes an ingenious contrivance for changing this angle at pleasure, in order to vary the velocity of the motion. Each vane is made to turn round a midrib, which stands out like a radius from the spindle, and the vane is moved by a stiff wire attached to one of the corners adjoining to the axle. These wires are attached to a ring which slides on the spindle like the spreader of an umbrella; and it is stopped on any part of the spindle by a pin thrust through a hole in the spindle and ring. We mention this briefly, it being easily understood by any mechanic, and but of little consequence, because the machine is not susceptible of much precision.

It is easy to see that an increase of the surface of the vanes will increase the power; therefore they should occupy the whole space of the circle, and not consist of four narrow arms like the sails of a windmill. It is better to make many narrow vanes than a few broad ones; as will appear plain to one well acquainted with the mode of impulse of fluids acting obliquely. We recommend eight or twelve at least; and each vane should be so broad, that when the whole is held perpendicular between the eye and the light, no light shall come through the fly, the vanes overlapping each other a very small matter. We also recommend the making them of stiff plate. Their weight contributes to the steady motion, and enables the fly, which has acquired a considerable velocity during a favourable position of things, to retain a momentum sufficient to pull round the spit while the heavy side of the meat is rising from its lowest position. In such a situation a light fly soon loses its momentum, and the jack flaggers under its load.

It is plain, from what has been said, that the fly should occupy the whole of that section of the vent where it is placed. The vent must therefore be brought to a round form in that place, that none of the current may pass uselessly by it.

It is an important question where the fly should be placed. If in a wide part of the vent, it will have a great surface, and act by a long lever; but the current in that place is slow, and its impulse weak. This is a fit subject of calculation. Suppose that we have it in our choice to place it either as it is drawn in the figure, or farther up at g, where its diameter must be one half of what it is at G. Since the same quantity of heated air passes through both sections, and the section g has only one-fourth of the area of the section G, it is plain that the air must be moving four times faster, and that its impulse is 16 times greater. But the surface on which it is acting is the fourth part of that of the fly G; the actual impulse therefore is only four times greater, supposing both flies to be moving with the same relative velocity in respect of the current; that is, the rim of each moving with the same portion of the velocity of the current. This will be the case when the small fly turns eight times as often in a minute as the large fly: for the air is moving four times as quick at g, and the diameter of g is one half of that of G. Therefore, when the small fly is turning eight times as quick as the great one, there is a quadruple impulse acting at half the distance from the axis. The momentum or energy therefore of the current is double. Therefore, supposing the pinion, wheel, and pulleys of both jacks to be the same, the jack with the small fly, placed in the narrow part of the vent, will be 16 times more powerful.

By this example, more easily understood than a general process, it appears that it is of particular importance to place the fly in an elevated part of the vent, where the area may be much contracted. In order still farther to increase the power of the machine, it would be very proper to lengthen the spindle still more, and to put another fly on it at a considerable distance above the first, and a third above this, &c.

As the velocity of the current changes by every change of the fire, the motion of this jack must be very unsteady. To render it as adjustable as may be to the particular purpose of the cook, the pulley E has several grooves of different diameters, and the spit turns more or less slowly, by the same motion of the fly, according as it hangs in the chain by a larger or smaller pulley or groove.

Such is the construction of the smoke-jack in its most simple form. Some are more artificial and complicated, having, in place of the pulleys and connecting chain, a spindle coming down from the horizontal axis BC. On the upper end of this spindle is a horizontal contrate wheel, driven by a pinion in place of the pulley C. On the lower end is a pinion, driving a contrate wheel in place of the pulley E. This construction is represented in fig. 6. Others are constructed more simply, in Fig. 6, the manner represented in fig. 7. But our first construction has great advantage in point of simplicity, and allows a more easy adjustment of the spit, which may be brought nearer to the fire or removed farther from it without any trouble; whereas, in the others, with a train of wheels and pinions, this cannot be done without several changes of pins and screws. The only imperfection of the pulley is, that by long use the grooves become slippery, and an ill-balanced joint is apt to hold back the spit, while the chain slides in the grooves. This may be completely prevented by making the grooves flat instead of angular (which greatly diminishes the friction), and furnishing them with short studs or pins which take into every third or fourth link of the chain. If the chain be made of the simplest form, with flat links, and each link be made of an exact length (making them all on a mould), the motion will be as easy as with any wheelwork, and without the least chance of slipping.

It is always of importance to avoid this slipping of the chain by balancing the loaded spit. For this purpose it will be extremely convenient to have what is called a balance-skewer. Let a part of the spit, immediately adjoining to the pulley, be made round, and let an arm be made to turn on it stiffly, so that it may be made fast in any position by a screw. Let a leaden ball be made to slide along this arm, with a screw to fasten it at any distance from the spit. When the meat is spitted, lay it on the racks, and the heaviest side will immediately place itself underneath. Now turn round the balance-skewer, so that it may point straight upwards, and make it fast in that position by the screw. Put the leaden ball on it, and slide it inwards or out- PLATE CCCCXCVII.

Fig. 1. Fig. 2. Fig. 3. Fig. 4. Fig. 5. Fig. 6. Fig. 7. Fig. 8. Fig. 9.

SOUNDING MACHINE

Engraved by W. & B. Dower Edin. wards till it exactly balances the heavy side, which will appear by the spit's remaining in any position in which it is put.

The greatest difficulty is to keep the machine in repair. The essential part of it, the first mover, the fly, and the pinion and wheel, by which its motion is transmitted to the rest of the machine, are situated in a place of difficult access, and where they are exposed to violent heat and to the smoke and foot. The whole weight of the fly, resting on the lower pivot I, must exert a great pressure there, and occasion great friction, even when this pinion is reduced to the smallest size that is compatible with the necessary strength. The pivot must be of hardened steel, tapered like an obtuse cone, and must turn in a conical socket, also of hardened steel or of bell-metal; and this seat of pressure and friction must be continually supplied with oil, which it consumes very quickly. It is not sufficient that it be from time to time smeared with an oiled feather; there must be an iron cup formed round the socket, and kept filled with oil. It is surprising how quickly it disappears; it soon becomes clammy by evaporation, and by the foot which gathers about it. The continued rubbing of the pivot and socket wears them both very fast; and this is increased by hard powders, such as sandy dust, that are hurled up by the rapid current every time that the cook fires the fire. These, getting between the rubbing parts, cause them to grind and wear each other prodigiously. It is a great improvement to invert these rubbing parts. Let the lower end of the spindle be of a considerable thickness, and have a conical hollow nicely drilled in its extremity. Let a blunt-pointed conical pin rise up in the middle of the oil cup, on which the conical hollow of the spindle may rest. Here will be the same steady support, and the same friction as in the other way; but no grinding dust can now lodge between the pivot and its socket: and if this upright pin be screwed up through the bottom of the cup, it may be screwed farther up in proportion as it wears; and thus the upper pivot g will never desert its hole, a thing which soon happens in the common way. We can say from experience, that a jack constructed in this way will not require the fifth part of the repairs of one done in the other way.

It is of importance that the whole be so put together as to be easily taken down, in order to sweep the vent, or to be repaired, &c. For this purpose, let the cross bar which carries the lower end of the upright spindle be placed a little on one side of the perpendicular line from the upper pivot hole. Let the cock which carries the oil cup and the pivot of the horizontal axis BC be screwed to one side of this cross bar, so that the centre of the cup may be exactly under the upper pivot hole. By this construction we have only to unscrew this cock, and then both axles come out of their places at once, and may be replaced without any trouble. We have sketched in fig. 8. the manner in which this may be done, where M represents a section of the lower cross bar. BCDE is the cock, fixed to the bar by the pins which go through both, with finger nuts a and b on the opposite side. F is the hard steel pin with the conical top i, on which the lower end l of the upright spindle AG rests, in the manner recommended as the best and most durable. The pivot of the horizontal axis turns in a hole at E the top of the cock.

After all, we must acknowledge that the smoke-jack is inferior to the common jack that is moved by a weight. It is more expensive at first, and requires more frequent repairs; its motion is not so much under command; it occasions foot to be thrown about the fire, to the great annoyance of the cook; and it is a great encumbrance when we would clean the vent.