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 2-1/2 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 23-1/4 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 2-1/2 feet square may be risked on a lower floor and 1-1/2 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 2-1/2 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.