Of Chimney Fireplaces. 531 



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 re- 

 duced. But it is absolutely necessary that these walls 

 should be carried up 5 or 6 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 in- 

 telligence enough to avail himself of the opening which 

 is formed in the back of the fireplace 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 be 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 Figs. 13 and 14, Plate XIII., where d shows the 

 top of the breast of the chimney) ; when this is the case, 



