118 



THE ARCH. 



fi feet. Divide AB into eleven equal partg, and call the 

 sixth from A, P; thus AP = -^iV and PB = tt of AB. Then 

 describe the arc of a circle passing through tlie points 

 D, P, E, having its centre in C. Also from the same centre 

 describe a parallel arc passing through A. Then the space 

 inclosed between the two arcs will be the strongest arch 

 that can be made within the limits of the platband, and all 

 the joints of the masonry or brickwork should be made to 

 radiate from the centre C* The true arch lies between the 

 circular arcs; the prolongation of the arch-stones below 

 that lino are merely suspensions from tho real arch above, 

 and do not add in any way to its strength. 



Let us now proceed to the consideration of tho strength of 

 the materials of which the arch has to bo built; namely, 

 stone or brick and cement. 



On first thought, it would be supposed that a good build- 

 ing stone would bo the strongest and best material for arch 

 building ; but tho determination of this question is usually 

 very much influenced by that of cost. If in building an arch 

 all the stones wore dressed to the true wedge form from 

 front to back, and then fully bedded IhrougJumt in cement, a 

 good stone would undoubtedly form the best arch; but, 

 practically, this cannot be done, for two reasons: Firstly, no 

 one will go to tho expense of using fully-dressed stones all 

 through the arch ; they will dress the stones on tlio face of 

 the arch from front to back for the sake of appearances, but 

 the middle parts of the arch, which really have to bear the 

 load, are usually filled with rough hacking, and are only 

 faced with dressed stone. The main body of tho stone arch 

 is therefore a mere face of dressed stones backed up with 



The circular arc, in so small a segment, represents tho efiuilibratcd 

 curve as nearly as is practicable. 



