Of Fcuitory Chimneys. 15 



The total pressure of the wind against the side of a cylinder is about 

 one-half of the total pressure against a diametral plane of that 

 cylinder. 



Let the figure represent a chimney, square or circular, and let it be 

 required to determine the conditions of stability 

 of a given bed-joint, D E. 



Let S denote the area of a diametral vertical 

 section of the part of the chimney above the 

 given joint, and p the greatest intensity of 

 pressure of the wind against a flat sui'face. Then 

 the total pressui'e of the wind against the 

 chimney will be sensibly 



P = ^:) S for a square chimney ; 



o 



P = ^ - for a round chimney ; 



(1) 



and its resultant may, without appreciable error, 



be assumed to act in a horizontal line through 



the centre of gravity of the vertical diametral 



section, C. Let H denote the height of that centre above the joint 



D E, then the moment of the pressure is 



HP 

 HP 



H 2; S for a square chimney . 

 — ^ — for a round chimney ; 



•(2.) 



and to this the least moment of stability of the portion of the chimney 

 above the joint D E, should be equal. 



For a chimney whose axis is vertical, the moment of stability is the 

 same in all directions. But few chimneys have their axes exactly 

 vertical ; and the least moment of stability is obviously that which 

 opposes a lateral pressure acting in that direction toward which the 

 chimney leans. 



Let G be the centre of gravity of tlte part of the chimney which is 

 above the joint D E, and 15 a point in the joint D E vertically below it ; 

 and let the line D E = < represent the diameter of that joint which 

 traverses the point B. Let q represent the ratio which the deviation 

 of B from the middle of the diameter D E bears to the length t of 

 that diameter. 



Let F be the limiting position of the centre of resistance of the joint 

 D E, nearest the edge of that joint towards which the axis of the 



