THE ARCH. 



Ill 



the chain are of the same length all through, and this brings 

 the rods nearer together towards the springing, whore the 

 chain becomes gradually more inclined. This, of course, 

 uiakos the vortical load a little too heavy as it approaches 

 tho springing ; but in an arch like this, of wide span and 

 small rise, it makes so little dilference that it is not worth 

 notice. 



Iloturning again to the " lino of thrust." If an additional 

 load be placed upon any part of the arch, the alteration in 

 the line of thrust is at once apparent in tho altered form 

 and tendon of tho cliain of the suspended arch, which 

 adjusts itself to the now equilibrated curvature ; but it is 

 not a,pparent in the constructed arch, notwithstanding that 

 tho lino of thrust is- similarly and equally altered in both. 

 The chain, in fact, gives us the means of knowing, mechani- 

 cally and precisely, what this alteration amounts to in the 

 forces aifecting the solid arch. 



The alteration in the " line of thrust," as passing through 

 a constructed arch without outwardly showing any visible 

 change, may be illustrated by a familiar example. Let us 

 suppose that a stone pillar has been standing lor some years 

 in a builder's j'ard. As it stands there, the " line of thrust " 

 IS vortical from the centre of gravity on to tho slab on 

 which it stands, the weight of the pillar giving the amount 

 01 tho thrust and the sizo of its base the margin of 

 Stability. Being out of tlio way, and handy for his purpose, 

 ^et us suppose a workman tilts a couple of planks against it; 

 the pillar makes no sign, but its "line of thrust " is altered 

 ^y the push of those planks, and brought a certain amount 

 nearer to tho opposite angle of the base. By-and-by the 

 workman tilts against it two more planks, and the thrust 

 line is brought another half-inch nearer to the angle of the 

 oase ; but yet there is no movement, for the point of thrust 



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