Depth, cast with them; so that when the Plates are put together, and the Joints luted, there is no Communication between the Air-Box and the Smoke. In the winding Passages of this Box, fresh Air is warm'd as it passes into the Room.

(v) The Front Plate is arch'd on the under Side, and ornamented with Foliages, &c.; it has no Ledges.

(vi) The Top Plate has a Pair of Ears, M N, answerable to those in the Bottom Plate, and perforated for the same Purpose: It has also a Pair of Ledges running round the under Side, to receive the Top Edges of the Front, Back, and Side Plates. The Air-Box does not reach up to the Top Plate by two Inches and half.

(vii) The Shutter is of thin wrought Iron and light, of such a Length and Breadth as to close well the Opening of the Fire-place. It is us'd to blow up the Fire, and to shut up and secure it a Nights. It has two brass Knobs for Handles, dd, and commonly slides up and down in a Groove, left, in putting up the Fire-place, between the foremost Ledge of the Side Plates, and the Face of the Front Plate; but some choose to set it aside when it is not in Use, and apply it on Occasion.

(viii) The Register, is also of thin wrought Iron. It is plac'd between the Back Plate and Air-Box, and can, by Means of the Key S be turn'd on its Axis so as to lie in any Position between level and upright.

The Screw-Rods, O P are of wrought Iron, about a third of an Inch thick, with a Button at Bottom, and a Screw and Nut at Top; and may be ornamented with two small Brasses screw'd on above the Nuts.

To put this Machine to work,

1. A false Back of four Inch-(or, in shallow small Chimneys, two-Inch-) Brick-Work is to be made in the Chimney, four Inches or more from the true Back: From the Top of this false Back, a Closing is to be made over to the Breast of the Chimney, that no Air may pass into the Chimney, but what goes under the false Back, and up behind it.

2. Some Bricks of the Hearth are to be taken up, to form a Hollow under the Bottom Plate; across which Hollow runs a thin tight Partition, to keep apart the Air entring the Hollow, and the Smoke; and is therefore plac'd between the Air-hole and Smoke-holes.

3. A Passage is made, communicating with the outward Air, to introduce that Air into the fore part of the Hollow under the Bottom Plate, whence it may rise thro' the Airhole into the Air-box.

4. A Passage is made from the back Part of the Hollow, communicating with the Flue behind the false Back: Through this Passage the Smoke is to pass.

The Fire-place is to be erected upon these Hollows, by putting all the Plates in their Places, and screwing them together.

Its Operation may be conceiv'd by observing the Plate entitled, Profile of the Chimney and Fire-place. (See Plate VI.) M The Mantel-piece, or Breast of the Chimney.

C The Funnel.

B The false Back and Closing.

E True Back of the Chimney.

T Top of the Fire-place.

F The Front of it.

A The Place where the Fire is made.

D The Air-Box.

K The Hole in the Side plate, thro' which the warm'd Air is discharg'd out of the Air-Box into the Room.

H The Hollow fill'd with fresh Air, entring at the Passage I, and ascending into the Air-Box thro' the Air-hole in the Bottom plate, near

G The Partition in the Hollow to keep the Air and Smoke apart.

P The Passage under the false Back and Part of the Hearth for the Smoke.

[The arrows show] the Course of the Smoke.

The Fire being made at A, the Flame and Smoke will ascend and strike the Top T, which will thereby receive a considerable Heat. The Smoke, finding no Passage upwards, turns over the Top of the Air-box, and descends between it and the Back Plate to the Holes in the Bottom Plate, heating, as it passes, both Plates of the Air-box, and the said Back Plate; the Front Plate, Bottom and Side Plates are also all heated at the same Time. The Smoke proceeds in the Passage that leads it under and behind the false Back, and so rises into the Chimney. The Air of the Room, warm'd behind the Back Plate, and by the Sides, Front, and Top Plates, becoming specifically lighter than the other Air in the Room, is oblig'd to rise; but the Closure over the Fire-place hindring it from going up the Chimney, it is forc'd out into the Room, rises by the Mantle-piece to the Cieling, and spreads all over the Top of the Room, whence being crouded down gradually by the Stream of newly-warm'd Air that follows and rises above it, the whole Room becomes in a short time equally warmed.

At the same Time the Air, warmed under the Bottom Plate and in the Air-Box, rises, and comes out of the Holes in