ART OF BUILDING. 251 



chimney must be raised up to the overtopping object, or 

 covered with a pot or cap as recommended by Mr Tred- 

 gold of England. 



His method consists in placing in the top of the chim- 

 ney a cap of brick or iron, formed in the following man- 

 ner : the top opening of the chimney is constructed in 

 size, according to the following rule. Divide seventeen 

 times the width of the grate or fire-place in inches, by the 

 square root of the height of the chimney in feet, the 

 quotient is the area of the opening at the top in square 

 inches. It should be understood however, that this 

 opening should never be less than Cinches in diameter, 

 and seldom more than 6J- ; the inside of the cap or 

 hood should be rounded off, and made as smooth as pos- 

 sible, so that the smoke may meet no obstacle. This 

 plan is very general in Boston, and gives a neat finish 

 to the top of the chimney, as well as prevents it from 

 smoking. 



Measurement of Masonry. Although it is not custom- 

 ary at the present day for masons to work by measure, 

 yet it may be useful to know the rules for measuring 

 stone and brick-work. We accordingly give a few of 

 them. 



Stone work is generally measured by the cubic foot or 

 perch, and almost every city has its particular rules as to 

 what parts of the work shall be measured in taking the 

 dimensions, therefore, no general rule can be given for 

 stone work. Brick work is either estimated by the rod 

 or by the thousand inches ; therefore, a rod of brick 

 work is 16^ feet square and 1^ bricks thick or 13^ inches ; 

 therefore, a rod of brick work is 306 cubic feet, while a 

 perch or rod of stone work in some places is only 3l 

 solid feet ; therefore, for brick work the rule is, multi- 

 ply the area of the wall by the number of half bricks in 

 thickness, and divide by 816 and the quotient is rods, and 

 the remainder, if any, feet. 



A rod of brick work laid in mortar will require 4500 

 bricks, a cubic yard 460. For paving, a yard of work, 

 82 paving bricks, or 38 bricks laid flat, are required. 



Masons and others are frequently called upon to esti- 

 mate the quantity of materials necessary to construct any 

 given work ; this may be easily done by the common rules 



