Fire-Places.
Reading edition
FROM THE DOWNINGSTON REPUBLICAN.
Fire-places, for warming rooms, have been for a long time in use; and the best plan for constructing them continues to be an interesting subject of investigation.
Whether or not fire-places and chimneys are of very ancient date, and mentioned by Virgil Appian, and Aristophanes, or whether they are of more modern invention, is not of much practical importance; and may be left to philosophers to determine. But as the proper construction of a chimney and fire-place is one of the greatest comforts of domestic life, I cannot doubt but that government will grant me a handsome premium for making public an unerring rule by which they can be so built as never to fail of drawing well, without emitting any smoke into the apartment. When the principle was first discovered by me, I wondered how it could so happen, that we had any defective chimnies amongst us; for I remembered to have heard that Dr. Franklin and count Rumford had devoted much attention to this subject, and must, of course, as I thought, have discovered a principle so plain and self-evident; and consequently, every mechanic must have learnt it, as not they only were interested in it, but every one of the civilized world.—After thinking of it in this way, I took an opportunity of inquiring into their opinions, and I find that the principle was never new to either of them; and as far as I know, I am the first and only discoverer of it. The principle consists simply in making the size of the flue bear a certain proportion to the size of the fire-place in front. To ascertain what would be the smallest proportion which the flue would bear to the front of the fire-place, would require some experiments. But it is probable that a fire-place 21⁄2 feet square in front, would draw well enough to carry up all the smoke, by a flue 12 by 14 inches, and probably by a one foot square: or even less may probably do.
If one foot square, would carry all the smoke of such a fire-place, the proportion would be 144 to 900, or a little more than 1⁄7, and a little less than 1⁄6. But for greater certainty, we will say 1⁄5 is the proper proportion, and that a flue to a fire-place 21⁄2 feet square, shall be 12 by 15 inches. If the front be twice that size, (3 feet 6 and near a half square) then the flue or chimney must be 12 by 30 inches, or near 19 inches square. If it be three times the first size, or near 4 feet 4 inches square, then the flue must be 12 by 45—15 by 36, or nearly 231⁄4 inches square. Or if it be four times the size of the first, or 5 feet square, the flue must be equal to nearly 27 inches square, and so accordingly, whatever may be the dimensions of the fire-place in front. I do not know that a lesser proportion would not do: and whatever experience may establish, as that best adapted to answer the end proposed, it will not invalidate the principle laid down, but confirm it. One thing, however, is true, that the smaller the fire-place is in proportion to the flue, the harder it will draw, and by being thus proportioned, it may have any degree of draught given to it at pleasure, from that of an air furnace to that which will scarcely draw up the smoke. Any one may convince himself of the truth of this doctrine, by taking a board and closing up the fire-place, downwards from the mantle, and in proportion as it diminishes by the board moving downwards, the power of its draught will be increased. Franklin was aware of this fact, but was ignorant of the principle on which it depended, as he supposed the height of the chimney, and not its dimensions governed the fact itself,—and hence mentions that the higher the chimney, the larger the opening may be: and that 21⁄2 feet square may be risked on a lower floor and 11⁄2 on the upper, &c. The common ten plate stove furnishes an example of the truth of this principle: when the large fire door is open, the stove will smoke, because the large door is an over proportion in size to the size of the pipe—but shut the large door and leave the small one open, the stove immediately draws like a furnace, because its area is smaller than the area of a section of the pipe. If it were true that 21⁄2 feet square was the largest size which could be made to draw well, it would indeed be an unfortunate fact, as we could never have a comfortable kitchen fire-place—but if the principle here laid down be true, we can, with equal certainty, build a fire-place ten feet wide and five high, or of greater dimensions if we choose.
The height of the chimney I believe to be a matter of no importance to the draught, and that a low one will draw as well as a high one. The worst smoking chimney that I ever saw was about 47 feet high, in my own house: it smoked because it was too small, and was cured by adding to it the flue of a fire-place directly above it; on account of which I had to lose a fire-place on the 2d floor. In this case the size of both chimneys was barely sufficient to vent the smoke of the lower one. The height is a subject which I have not investigated, but it strikes me that a low chimney is most favourable to drawing well: if any one want an explanation on it, I will give it again; but it cannot be of much practical importance. The fashion of a fire-place is of no importance to the draught: it may be made to please the fancy of the builder. I, however, should prefer one something near count Rumford's plan. The throat of the chimney may be contracted or not at pleasure; but for beauty and advantage of heat, I should prefer having the back drawn forward and the throat narrowed, more especially in a low fire-place. There may be local contingent circumstances connected with chimneys, which cause them to smoke, and must have appropriate remedies: but such do not effect the general principles here laid down.
Smoke, from fires, is naturally carried upward by the heated air, which is specifically lighter than the surrounding atmosphere, and consequently ascends, carrying the smoke with it, and if it meet with no resistance, will pass up the chimney; but if that be too small to vent it, it regurgitates, as it were, into the apartment; which can only be remedied on the principle heretofore laid down.
It might be asked, why will not a small chimney vent all the smoke of a small fire, in a large fire-place? Two reasons may be assigned: First, the heat of such fire cannot produce a brisk enough current of rarified air to carry the smoke, and that which is heated, not having free vent, it whirls in eddies into the apartment. Secondly, smoke, in itself, is specifically heavier than atmospheric air; consisting of aqueous vapour, carbonic acid, and oxid of carbon, and coming in contact with air not much heated, and parting with a portion of its own heat, it has then no disposition, in itself, to ascend. But this would lead to a discussion foreign to my present subject.
Yours, &c. N.
Samuel Sprigg, Esq. has been elected by the legislature, governor of Maryland; and Thomas Mane Randolph governor of Virginia, in the room of Mr. Preston, whose constitutional term of office has expired.