degrees, -- the angle which the plane of the back of a Chimney Fire-place ought to make with the plane of its covings. -- And a bevel m, n, being made to this angle with thin slips of hard wood, this little instrument will be found to be very useful in marking out on the hearth, with chalk, the plans of the walls which are to form the covings of Fire-places.

As Chimneys which are apt to smoke will require the covings to be placed less obliquely in respect to the back than others which have not that defect, it would be convenient to be provided with several bevels; -- three or four, for instance, forming different angles. -- That already described, which may be called No. 1. will measure the obliquity of the covings when the Fire-place can be made of the most perfect form: -- another No. 2. may be made to a smaller angle, d c e, -- and another No. 3. for Chimnies which are very apt to smoke at the still smaller angle d c i. -- Or a bevel may be so contrived, by means of a joint, and an arch, properly graduated, as to serve for all the different degrees of obliquity which it may ever be necessary to give to the covings of Fire-places.

Another point of much importance, and particularly in Chimneys which are apt to smoke, is to form the throat of the Chimney properly, by carrying up the back and covings to a proper height. This, workmen are apt to neglect to do, probably on account of the difficulty they find in working where the opening of the canal of the Chimney is so much reduced. -- But it is absolutely necessary that these walls should be carried up five or six inches at least above the upper part of the breast of the Chimney, or to that point where the wall which forms the front of the throat begins to rise perpendicularly. -- If the workman has intelligence enough to avail himself of the opening which is formed in the back of the Fire-place to give a passage to the Chimney-sweeper, he will find little difficulty in finishing his work in a proper manner.

In placing the plumb-line against the breast of the Chimney, in order to ascertain how far the new back is to be brought forward, great care must be taken to place it at the very top of the breast, where the canal of the Chimney begins to rise perpendicularly; otherwise, when the plumb-line is placed too low, or against the slope of the breast, when the new back comes to be raised to its proper height, the throat of the Chimney will found to be too narrow.

Sometimes, and indeed very often the top of the breast of a Chimney lies very high, or far above the fire (see the figures 13 and 14, where d shows the top of the breast of the Chimney); when this is the case it must be brought lower, otherwise the Chimney will be very apt to smoke. -- So much has been said in the First Chapter of this Essay of the advantages to be derived from bringing the throat of a Chimney near to the burning fuel, that I do not think it necessary to enlarge on them in this place, -- taking it for granted that the utility and necessity of that arrangement have already been made sufficiently evident; -- but a few directions for workmen, to show them how the breast (and consequently the throat) of a Chimney can most readily be lowered, may not be superfluous.

Where the too great height of the breast of a Chimney is owing to the great height of the mantle, (see fig. 13,) or, which is the same thing, of the opening of the Fire-place in front, which will commonly be found to be the case; the only remedy for the evil will be to bring down the mantle lower; -- or rather, to make the opening of the Fire-place in front lower, by throwing across the top of this opening, from one jamb to the other, and immediately under the mantle, a very flat arch; -- a wall of bricks and mortar, supported on straight bars of iron; -- or a piece of stone (h, fig. 13). -- When this is done, the slope of the old throat of the Chimney, or of the back side of the mantle, is to be filled up with plaster, so as to form one continued flat, vertical, or upright plane surface with the lower part of the wall of the canal of the Chimney, and a new breast is to be formed lower down, care being taken to round it off properly, and make it finish at the lower surface of the new wall built under the mantle; -- which wall forms in fact a new mantle.

The annexed drawing fig. 13, which represents the section of a Chimney in which the breast has been lowered according to the method here described, will show these various alterations in a clear and satisfactory manner. In this figure, as well as in most of the others in this Essay, the old walls are distinguished