30 



Proceedinqs of the Royal Irish Academij. 



As the normal load at the crown is no gi-eater, the raduis of the line of stress 

 must increase at A, and begin at a point lower down in that joint. The 

 sharpest point on the modified line of stress will be at the 30° joint, and 

 high up in that joint, and that 30° joint, together with the 4315° joint, are 

 the tico joints of ruptnre. That the masonry ring has been chosen of a thick- 



C/BCUIAR ARCH. 



GRAPHICAL CONSTRUCT/0/V / ; \ . -. ^-, ^ I 



OF THE HORIZONTAL COM-' ■ ^ 

 -JUCATE LOAD. 



jy«?= (/ri^ 6^$!^. / 





FC_ ,__,>i-l 



so Feet 



Fig. oa. 



ness sufficient for eqiulihriiim is determined if the modified line of stress lie 

 within the middle third of the three joints, the crown, and these two joints of 

 rupture. To be of the most economical thickness the centre of stress at the 

 crown (cheeks of the keystone) should be at the v.pper limit of ihe middle 

 third, and at the lower limit in the 30° joint. 



