And this leads us to consider another important point respecting open Fire-places, and that is, the width which it will, in each case, be proper to give to the back.--In Fire-places as they are now commonly constructed, the back is of equal width with the opening of the Fire-place in front;--but this construction is faulty on two accounts.--First, in a Fire-place, so constructed, the sides of the Fire-place, or COVINGS, as they are called, are parallel to each other, and consequently ill-contrived to throw out into the room the heat they receive from the fire in the form of rays;--and secondly, the large open corners which are formed by making the back as wide as the opening of the Fire-place in front occasion eddies of wind, which frequently disturb the fire, and embarrass the smoke in its ascent in such a manner as often to bring it into the room.--Both these defects may be entirely remedied by diminishing the width of the back of the Fire-place. --The width which, in most cases, it will be best to give it, is ONE THIRD of the width of the opening of the Fire-place in front.--But it is not absolutely necessary to conform rigorously to this decision, nor will it always be possible.--It will frequently happen that the back of a Chimney must be made wider than, according to the rule here given, it ought to be.--This may be, either to accommodate the Fire-place to a stove, which being already on hand, must, to avoid the expense of purchasing a new one, be employed; or for other reasons;--and any small deviation from the general rule will be attended with no considerable inconvenience.--It will always be best, however, to conform to it as far as circumstances will allow. Where a Chimney is designed for warming a room of a middling size, and where the thickness of the wall of the Chimney in front, measured from the front of the mantle to the breast of the Chimney, is nine inches, I should set off four inches more for the width of the throat of the Chimney, which, supposing the back of the Chimney to be built upright, as it always ought to be, will give thirteen inches for the depth of the Fire-place, measured upon the hearth, from the opening of the Fire-place in front, to the back.--In this case thirteen inches would be a good size for the width of the back; and three times thirteen inches, or thirty-nine inches, for the width of the opening of the Fire-place in front; and the angle made by the back of the Fire-place and the sides of it, or covings, would be just 135 degrees, which is the best position they can have for throwing heat into the room.