No inconvenience of any importance will arise from making the obliquity of the covings LESS than what is here recommended; but many cannot fail to be produced by making it much greater;-- and as I know from experience that workmen are very apt to do this, I have thought it necessary to warn them particularly against it. Fig. 11. shows how the width and obliquity of the covings of a Chimney are to be accommodated to the width of the back, and to the opening in front and depth of the Fire-place, where the width of the opening of the Fire-place is less than three times the width of the new back. As all those who may be employed in altering Chimneys may not, perhaps, known how to set off an angle of any certain numbers of degrees,--or may not have at hand the instruments necessary for doing it,--I shall here show how an instrument may be made which will be found to be very useful in laying out the work for the bricklayers. Upon a board about 18 inches wide and four feet long, or upon the floor or a table, draw three equal squares A, B, C, fig. 12. of about 12 or 14 inches each side, placed in a strait line, and touching each other.--From the back corner c of the center square B, draw a diagonal line across the square A, to its outward front corner f, and the adjoining angle formed by the lines d c and c f will be equal to 135 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.