11'6 



THE ABCH. 



into account. This would thoroforo make tlio total load at 

 the crown 9-32 cwt. on the square foot ; and this multiplied 

 by the radius, 110 feet, would give 51-26 tons on the foot in 

 width of the arch, rendering necessary a thickness of 17-1 

 inches— that is, an additional thickness of 4-3 inches ; but, 

 as the original curvature would have been restored, this 

 takes ofi" the dejlection of the " line of thrust " before alluded 

 to,— a loss trying load than that of the single engine,— i/m 

 tries the oquilihratum of the arch, the other only the crushing 

 strength of its materials. 



Before we leave this question of general remarks on the 

 " line of thrust " it may be well to consider the general 

 stability of arches, whether built of the true curvature or 

 not, from another point of view ; namely, that of what may 

 be called the destructive load. 



Let us suppose that, for experimental purposes, we are 

 going to load a woU-built arch until wo break it down ; and 

 let us further suppose that, with this object in view, we 

 pile up the load continually upon the crown of the arch 

 until we attain that result ; the arch having been built, wo 

 suppose, to carry safely a given load, and loaded up to that 

 extent before wo begin to add gradually the destructive 

 load. Wo may have drawn beforehand the lines which we 

 Iiavo called the necessary thickness ; and as each increment 

 to the load is added, we may add also to the curvature and 

 thickness of this necessary arch the theoretical amounts duo 

 to the loads just imposed, which, being on the crown, will 

 keep on raising and sharpening the curvature of the lino of 

 thrust, as has boon described in the former case, until at 

 last the curvature gets outside the lines of the arch. Wo 

 then, in accordance with our theory, expect that tho arch 

 will fall from dislocation or want of balance, and not from 

 the crushing of the materials, because tho additional loads 



