Of Factory Chimneys. 17 



This becomes, when applied to square chimneys, 

 HpS = (i-5')W«; 

 and when applied to round chimneys, 





(G.) 



The following approximate formulae, deduced from these equations, 

 are useful in practice. 



Let B be the mean thickness of brickwork above the joint D E under 

 consideration, and b the thickness to which that brickwork would be 

 reduced if it were spread out flat upon an area equal to the external 

 area of the chimney. That reduced thickness is given with sufficient 

 accuracy by the formula, 



' = b(i-?)^ P) 



but in most cases the difference between h and B may be neglected. 



Let w be the weight of a cubic foot of brickwork ; being from 112 

 lbs. to 120 lbs. Then we have very nearly. 



For square chimneys, W = 4^«6S; ) /o\ 



For round chimneys, W = 3'14 i« 6 S ; ) 



which values being substituted in the equations 6, give the following 

 formulae : — 



1 

 iT^/ ' ^..(9.) 



For round chimneys, H jj> = (1 -57 — 6-28 g') wbt. 



For square chimneys, Hjt?= ( — — 4iq'\wht; 



These formulae serve two purposes ; first, when the greatest intensity 

 of the pressure of the wind, ^>, and the external form and dimensions 

 of a proposed chimney are given, to find the mean reduced thickness of 

 brickwork, h, required above each bed-joint, in order to insure stability ; 

 and, secondly, when the dimensions and form, and the thickness of the 

 brickwork of a chimney are given, to find the greatest intensity of 

 pressure of wind which it will sustain with safety. 



The shell of a chimney consists of a series of divisions, one above 

 another, the thickness being uniform in each division, but diminishing 

 upwards from division to division. The bed-joints between the divi- 

 sions, where the thickness of brickwork changes (including the bed- 

 joint at the base of the chimney), have obviously less stability than the 

 intermediate bed-joints ; hence it is only to the former set of joints 

 that it is necessary to apply the formulai. Those formulae have been 



Vol. IV.— No. 1. d 



