Benjamin Franklin Papers

From Benjamin Franklin to James Bowdoin, 2 December 1758

To James Bowdoin2

MS not found; reprinted from Benjamin Franklin, Experiments and Observations on Electricity (London, 1769), pp. 369–74.

The problem of “augmenting the Benefit of Fire”3 and cutting expenditures for fuel compelled Franklin’s attention throughout his life. The Pennsylvania fireplace represents his earliest and most famous attempt to solve it. A less familiar contrivance, conceived for the same purpose, was the “simple machine” which he described in this letter to Bowdoin. Either Franklin did not give Bowdoin a full description of this “machine,” which he preferred to call his “Sliding Plate,”4 and we should call a damper, or he improved it in the years immediately following 1758, for in describing it to Sir Alexander Dick in 1762 he explained that the sliding plate was hinged. When it was pulled forward to the hinge the front section could be dropped to “hang perpendicular,” thereby closing the upper part of the fireplace opening and permitting the fire to be “speedily blown up” by the increased draft.5 For a time, Franklin’s sliding plate enjoyed a considerable vogue in London and “many Hundreds” were “set up in Imitation of it.”6 He recommended it to his Scottish friends, Dick and Lord Kames, and mentioned it in 1785 in his celebrated essay “On the Causes and Cure of Smoky Chimneys.”7

London, Dec. 2, 1758.

Dear Sir,

I have executed here an easy simple contrivance, that I have long since had in speculation, for keeping rooms warmer in cold weather than they generally are, and with less fire. It is this. The opening of the chimney is contracted, by brick-work faced with marble slabs, to about two feet between the jambs, and the breast8 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; drawing it out, so as to leave a space between its farther edge and the back, of about two inches; this space is sufficient for the smoke to pass; and so large a part of the funnel being stopt by the rest of the plate, the passage of warm air out of the room, up the chimney, is obstructed and retarded, and by that means much cold air is prevented from coming in through crevices, to supply its place. This effect is made manifest three ways. First, 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. Secondly, 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. Thirdly, 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. Several of my acquaintance having seen this simple machine in my room, have imitated it at their own houses, and it seems likely to become pretty common. I describe it thus particularly to you, because I think it would be useful in Boston, where firing is often dear.

Mentioning chimneys puts me in mind of a property I formerly had occasion to observe in them, which I have not found taken notice of by others;9 it is, that in the summer time, when no fire is made in the chimneys, there is, nevertheless, a regular draft of air through them; continually passing upwards, from about five or six o’clock in the afternoon, till eight or nine o’clock the next morning, when the current begins to slacken and hesitate a little, for about half an hour, and then sets as strongly down again, which it continues to do till towards five in the afternoon, then slackens and hesitates as before, going sometimes a little up, then a little down, till in about half an hour it gets into a steady upward current for the night, which continues till eight or nine the next day; the hours varying a little as the days lengthen and shorten, and sometimes varying from sudden changes in the weather; as if, after being long warm, it should begin to grow cool about noon, while the air was coming down the chimney, the current will then change earlier than the usual hour, &c.

This property in chimneys I imagine we might turn to some account, and render improper, for the future, the old saying, as useless as a chimney in summer. If the opening of the chimney, from the breast down to the hearth, be closed by a slight moveable frame, or two in the manner of doors, covered with canvas, that will let the air through, but keep out the flies; and another little frame set within upon the hearth, with hooks on which to hang joints of meat, fowls, &c. wrapt well in wet linen cloths, three or four fold, I am confident that if the linen is kept wet, by sprinkling it once a day, the meat would be so cooled by the evaporation, carried on continually by means of the passing air, that it would keep a week or more in the hottest weather. Butter and milk might likewise be kept cool, in vessels or bottles covered with wet cloths. A shallow tray, or keeler, should be under the frame to receive any water that might drip from the wetted cloths. I think, too, that this property of chimneys might, by means of smoak-jack vanes,1 be applied to some mechanical purposes, where a small but pretty constant power only is wanted.

If you would have my opinion of the cause of this changing current of air in chimneys, it is, in short, as follows. In summer time there is generally a great difference in the warmth of the air at mid-day and midnight, and, of course, a difference of specific gravity in the air, as the more it is warmed the more it is rarefied. The funnel of a chimney being for the most part surrounded by the house, is protected, in a great measure, from the direct action of the sun’s rays, and also from the coldness of the night air. It thence preserves a middle temperature between the heat of the day, and the coldness of the night. This middle temperature it communicates to the air contained in it. If the state of the outward air be cooler than that in the funnel of the chimney, it will, by being heavier, force it to rise, and go out at the top. What supplies its place from below, being warmed, in its turn, by the warmer funnel, is likewise forced up by the colder and weightier air below, and so the current is continued till the next day, when the sun gradually changes the state of the outward air, makes it first as warm as the funnel of the chimney can make it, (when the current begins to hesitate) and afterwards warmer. Then the funnel being cooler than the air that comes into it, cools that air, makes it heavier than the outward air; of course it descends; and what succeeds it from above, being cool’d in its turn, the descending current continues till towards evening, when it again hesitates and changes its course, from the change of warmth in the outward air, and the nearly remaining same middle temperature in the funnel.

Upon this principle, if a house were built behind Beacon-hill, an adit2 carried from one of the doors into the hill horizontally, till it met with a perpendicular shaft sunk from its top, it seems probable to me, that those who lived in the house, would constantly, in the heat even of the calmest day, have as much cool air passing through the house, as they should chuse; and the same, though reversed in its current, during the stillest night.

I think, too, this property might be made of use to miners; as where several shafts or pits are sunk perpendicularly into the earth, communicating at bottom by horizontal passages, which is a common case, if a chimney of thirty or forty feet high were built over one of the shafts, or so near the shaft, that the chimney might communicate with the top of the shaft, all air being excluded but what should pass up or down by the shaft, a constant change of air would, by this means, be produced in the passages below, tending to secure the workmen from those damps which so frequently incommode them. For the fresh air would be almost always going down the open shaft, to go up the chimney, or down the chimney to go up the shaft. Let me add one observation more, which is, That if that part of the funnel of a chimney, which appears above the roof of a house, be pretty long, and have three of its sides exposed to the heat of the sun successively, viz. when he is in the east, in the south, and in the west; while the north side is sheltered by the building from the cool northerly winds. Such a chimney will often be so heated by the sun, as to continue the draft strongly upwards, through the whole twenty-four hours, and often for many days together. If the outside of such a chimney be painted black, the effect will be still greater, and the current stronger. I am, dear Sir, yours, &c.


[Note numbering follows the Franklin Papers source.]

2Boston merchant and scientific correspondent of BF; see above, IV, 69 n.

3“An Account of the New-Invented Fire-Places,” above, II, 422.

4BF to Lord Kames, Feb. 28, 1768, Scottish Record Office; to Jan Ingenhousz on smoky chimneys, Aug. 28, 1785, N.Y. Pub. Lib.

5In producing this effect BF was adapting to some extent the shutter from his Pennsylvania fireplace; see above, II, 431, 434.

7The next to last sentence and the first part of the last should be read as correcting a statement in the headnote on the Pennsylvania fireplace, above, II, 420. There the editors said that it was the fireplace which was imitated in London by the hundreds and that BF sent a model to Sir Alexander Dick and recommended it to Lord Kames. It now appears most probable, however, that it was his sliding damper that attracted most attention in Great Britain, and that he installed it in his Craven Street lodgings in 1757 or 1758 after contending with the coal-burning fires of London, so different in many of their characteristics from the wood fires of Philadelphia for which he had designed the Pennsylvania fireplace. Furthermore, as he explained to Kames in 1768, the version of the Pennsylvania fireplace produced in England had been so “improved” that he declined to recommend it to friends.

8The wall between the chimney flue and the room and above the fireplace opening.

9Apparently BF had not committed his observations to paper; this is the earliest of his writings on chimney ventilation that has been found.

1A smoke jack is a machine for turning a roasting-spit using the power generated by the ascending current of heated air in the flue. Vanes in the jack’s shaft set in the flue are turned by the current and a simple transmission system conveys the motion to the spit before the fire.

2A tunnel. Bowdoin had acquired his mansion on Beacon Hill, Boston, in 1756. Justin Winsor, The Memorial History of Boston (Boston, 1882), II, 522.

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