throat of a Chimney produces the same effects; and this is one reason, among many others, which have induced me to disapprove of register stoves.

The current of air, which, passing under the mantle, gets into the Chimney, should be made gradually to bend its course upwards, by which means it will be quietly with the ascending current of smoke, and will be less likely to check it, or force it back into the room. -- Now this may be effected with the greatest ease and certainty, merely by rounding off the breast of the Chimney or back part of the mantle, instead of leaving it flat, or full of holes and corners; and this of course ought always to be done.

I have hitherto given no precise directions in regard to the height to which the new back and covings ought to be carried: -- This will depend not only on the height of the mantle, but also, and more especially, on the height of the breast of the Chimney, or of that part of the Chimney where the breast ends and the upright canal begins. -- The back and covings must rise a few inches, five or six for instance, higher than this part, otherwise the throat of the Chimney will not be properly formed: -- but I know of no advantages that would be gained by carrying them up still higher.

I mentioned above, that the space between the walls which form the new back and covings, and the old back and sides of the Fire-place, should be filled up: -- but this must not be understood to apply to the space between the wall of dry bricks, or the tile which closes the passage for the Chimney-sweeper, and the old back of the Chimney; for that space must be left void, otherwise, though this tile (which at most will not be more than two inches in thickness,) were taken away, there would not be any room sufficient for him to pass.

In forming this door-way, the best method of proceeding is to place the tile or flat piece of stone destined for closing it, in its proper place; and to build round it, or rather by the sides of it; taking care not to bring any mortar near it, in order that it may be easily removed when the door-way is finished. -- With regard to the rabbet which should be made in the door-way to receive it and fix it more firmly in its place, this may either be formed at the same time when the door-way is built, or it may be made after it is finished, by attaching to its bottom and sides, with strong mortar, pieces of thin roof tiles. Such as are about half an inch in thickness will be best for this use; if they are thicker, they will diminish too much the opening of the door-way, and will likewise be more liable to be torn away by the Chimney-sweeper in passing up and down the Chimney.

It will hardly be necessary for me to add, that the tile, or flat stone, or wall of dry bricks, which is used for closing up the door-way, must be of sufficient height to reach quite up to a level with the top of the walls which form the new back and covings of the Chimnies.

I ought, perhaps, to apologize for having been so very particular in these description and explanations, but it must be remembered that this chapter is written principally for the information of those who, having had few opportunities of employing their attention in abstruse philosophical researches, are not sufficiently practised in these intricate investigations, to seize, with facility, new ideas; -- and consequently, that I have frequently been obliged to labour to make myself understood.

I have only to express my wishes that my reader may not be more fatigued with this labour than I have been; -- for we shall them most certainly be satisfied with each other. -- But to return once more to the charge.

There is one important circumstance respecting Chimney Fire-places, destined for burning coals, which still remains to be farther examined; -- and that is the Grate.

Although there are few grates that may not be used in Chimneys constructed or altered upon the principles here recommended, yet they are not, by any means, all equally well adapted for that purpose. -- Those whose construction is the most simple, and which of course are the cheapest, are beyond comparison the best, on all accounts. -- Nothing being wanted in these Chimnies but merely a grate for containing the coals, and in which they will burn with a clear fire; -- and all additional apparatus being, not only useless,