Benjamin Franklin Papers

From Benjamin Franklin to Sir Alexander Dick, 21 January 1762

To Sir Alexander Dick

ALS: New York Public Library; copy (incomplete):3 Scottish Record Office

London, Jan. 21. 1762

Dear Sir,

It gives me Pleasure to learn, by yours of Nov. 12. that my young Friend Mr. Morgan has render’d himself agreable to you, and that your Health and Eyes were much better.4

I sent some time since to Mr. Dalrymple5 one of my Machines for your Chimney, who readily paid the Smith’s Bill for the same. But now, on discoursing with some Gentlemen from Edinburgh, I am in doubt whether it is what you intended and expected. If not, pray let me know, that I may endeavour to procure for you the Thing that you desire.

However, let me tell you, that after more than 20 Years Experience of my own Contrivances and those of others, for the Warming of Rooms, and much Thought on the Subject, I am of Opinion, that This, all Circumstances considered, is by far the best for common Use. You will judge of it, when I have explain’d the Manner of Fixing it up, and its Operation.6

It is a thin Iron Plate sliding in a grooved Frame of Iron. The Opening of your Chimney I suppose is wider than this Plate with its Frame is long, and deeper than it is wide: In which Case, your Mason is to contract the Opening, by raising within it two Jambs of Brickwork about 3 Feet high, and at such a Distance from each other, that the Frame and Plate being laid on them may rest firmly, and be fix’d by additional Brickwork above upon the Jambs, and across from Jamb to Jamb over the Frame, so as to close the Opening above the Frame. This new Brickwork may be fac’d with Dutch Tiles, Stone or Marble at your Pleasure. This Work is to be plac’d so far back in the Chimney, that when the Plate is close thrust in, the Chimney is quite stopt up, so as to prevent all Passage of Air up or down. Then when you make a Fire, the Plate is to be drawn out so far only as to admit a Passage for all the Smoke; which will be one, two, or three Inches, at different Times, according to the Coldness of the Weather, and the Strength of Draft in your Chimney. If at any time, you would have the Fire speedily blown up, the Plate is to be drawn out as far as the Hinge, and let down so as to hang perpendicular, which enlarging the Passage above the Fire, and contracting it before, produces the Effect by occasioning a stronger Current of Air where it is required for the purpose.7

The Principles of this Construction are these. Chimney Funnels are made much larger than is necessary for Conveying the Smoke. In a large Funnel, a great Quantity of Air is continually ascending out of the Room, which must be supply’d thro’ the Crevices of Doors, Windows, Floors, Wainscots, &c. This occasions a continual Current of cold Air from the extream Parts of the Room to the Chimney, which presses the Air warm’d by the direct Rays of the Fire into the Chimney, and carries it off, thereby preventing its diffusing itself to warm the Room. By contracting the Funnel with this Plate, the Draft of Air up the Chimney is greatly lessen’d, and the Introduction of cold Air thro’ the Crevices to supply its Place is proportionably lessen’d. Hence the Room is more uniformly warm’d, and with less Fire; and the Current of cold Air towards the Chimney being lessen’d, it becomes much more comfortable Sitting before the Fire.

That the Draft of cold Air into the Room is lessen’d by this Plate maybe demonstrated by several easy Experiments. When you have a lively Fire burning, and the Plate as far in as it will bear to be without Stopping the Smoak, set the Door open about ½ an Inch, and hold your Hand against the Crevice; you will then feel the cool Air coming in, but slowly and weakly compar’d with what you will feel, if, while your Hand continues so plac’d, another Person suddenly draws out the Plate. The stronger Pressure of the outward Air into the Room, will, when the Plate is drawn out, push the Door more strongly; and being shut, the Rushing of the Air thro’ Crevices makes a louder Noise.

Since I first us’d this Contrivance in the Chimneys of my Lodging here, many Hundreds have been set up in Imitation of it, in and about this City, and they have afforded general Satisfaction. Simplicity, Cheapness, and Easy Execution, have all contributed to recommend it. Then it is no Obstruction to the Sweeping of the Chimney, is attended with no ill Smells, and in summer serves the Purpose of a Chimney-Board, by closing the Chimney entirely.

It has indeed been mistaken by some, as intended for the Cure of Smoaky Chimneys. But that is not to be expected from it, except in two Cases, viz. where the Chimney smokes because the Opening is too large; or where the Room is so tight and the Funnel so big, that all the Crevices together do not admit Air enough to supply the Draft: In these Cases it is of Service. But Chimneys often Smoke from other Causes, and must have other Remedies.

Possibly where a Chimney smokes from Wind sometimes blowing down, it may also be of some Service, the Push of the heated Air upwards being stronger in its narrow Passage. But in this Case I have had no Opportunity of seeing it try’d.

If you are desirous of obtaining still more Heat in your Room from the same Fire, I would recommend lining your Jambs with coving Plates of polish’d Brass. They throw a vast deal of Heat into the Room by Reflection. I have done my Parlour Chimney in that Manner with very good Effect. The Plates are thin, and the Expence of the two, but about twenty-five Shillings.

Please to acquaint your Friend Dr. Hope,8 that I am about returning to America this Summer, and will send him free of Charge for Postage in America, any Letters containing Leaves of Plants or small Parcels of Seeds that shall be committed to my Care by any of his or your medical Friends there.

My Son joins in best Wishes for you and your Children. Our Compliments to the eldest, who proves an excellent Secretary for you. Be so good as to present our cordial Regards to Lord Kaims when you see him. I shall write to him shortly, being much in his Debt. With the greatest Esteem, I am, Dear Sir, Your most obedient humble Servant

B Franklin

Sir Alexr. Dick

[Note numbering follows the Franklin Papers source.]

3The second MS, described at the top as “Copy of part of a Letter” from BF to Dick, with the date, begins at the second paragraph of the ALS and omits the final two paragraphs, the complimentary close, and the signature. It is endorsed “Copy Letter from Dr. Franklin about his Chimney Machines. 21 January 1762.”

4Dick’s letter of November 12 not found; it probably acknowledged a letter from BF of about Oct. 21, 1761, also not found, introducing the young medical student John Morgan; see above, IX, 373–7.

5Not identified.

6BF repeats here much of what he had written James Bowdoin, Dec. 2, 1758 (above, VIII, 194–6) about his “Sliding Plate” or damper to control the velocity of smoke and heated air passing up a chimney. Indication that the plate was to be hinged so that part could be let down to cover much of the fireplace opening is an addition to the description he had given Bowdoin.

7A similar though sometimes dangerous method of inducing temporarily a strong draft in a fireplace, somewhat more common in Great Britain than in the United States, is to open out and hold a newspaper across the upper part of the fireplace opening. Constriction of the incoming air to the lowest segment of the opening increases the draft through the coals or wood and promotes more active burning of a sluggish fire. Perhaps BF knew of this method and characteristically developed an apparatus to achieve the same result more easily and safely.

8John Hope (1725–1786), physician and botanist, professor of botany at the University of Edinburgh, 1761, and superintendent of the botanical garden. He was associated with Dick in the cultivation of Tartarian rhubarb. Correspondence with John Bartram, developing out of this contact with BF, is printed in Darlington, Memorials, pp. 432–6.

Index Entries