122 



THE ARCH. 



100,000 tons, and the 50-ton engine passing over it wonld 

 have a very slight effect on so heavy a structure. The 

 deflection at the centre would be less th.an half an inch, 

 therefore the nine inches of margin would bo ample. 



With an arch 6 feet in thickness, the span might be 400 

 feet. These figures are calctilatod on the assumption that 

 the safe load on brickwork is limited to one-eighth of the 

 crushing weight on cement. If what is considered the 

 safe load should be extended to one-quarter of the crushing 

 weight, as is done in the case of iron, then the same thick- 

 nesses of arch given above would bo sufficient for twice the 

 span mentioned. 



The chief argument against groat spans lies in the cost of 

 the centering. But a complete centering from springing to 

 springing would be very seldom wanted ; it is generally 

 quite easy to divide the support of the centering into a 

 number of sections — the more the better within certain 

 limits. Not only for saving the groat cost, but also because 

 if the span is a wide one the centering must be framed to- 

 gether with timbers of moderate length. — Now, when it is 

 recollected that this centering has to bear the weight of the 

 whole arch without shrinking (for if any shrinking occurs 

 the curvature of the arch must be immediately impaired), 

 how can wo expect that the struts in a self-supported, 

 framed centering will not bo driven further into their 

 housings, and all the joints throughout the centering closer 

 together by the imposition of this enormous load upon 

 centres that have not boon loaded before? It would 

 therefore be better to have a lino of props at every 

 20 feet if a good base can be got for them. It would 

 save the large cost of a framed centering, and we should 

 have one that could not sink under tho load. Centering 

 with a number of intermediate siipports would therefore 



