1818.] across the Mersey, at Runcorn. 27 



2. To support this comparatively greater strain of the catena- 

 rian arch, a proportionally stronger fulcrum is required ; and if it 

 is attempted to lighten the strain by raising the fulcrum to a 

 considerable height, it must evidently be attended with veiy 

 considerable danger from the vibratory motion of such an enorr 

 mous weight on a high tower, however strong. At all events 

 it must be attended with a greater risk of danger than in the use 

 of the sort of fulcrums in the design submitted, where the 

 strain is communicated in separate portions, and is easily sub- 

 jected to calculation. 



3. Resiliency, or undulatory motion, is much more certain of 

 taking place in a road suspended from a catenarian arch, than it 

 is in the case of a road suspended agreeably to the construction 

 submitted. Every inverted catenarian arch, however loaded, must 

 be liable to have its equilibrium disturbed by a certain increase of 

 weight in any one part. The point where the weight is applied 

 will sink, and those adjoining will rise, and so will every part of 

 the road, as the load passes along successively, and thus an un- 

 dulating motion is produced. This motion proceeds to the pier, 

 on which the road rests ; it there meets with resistance, increasing 

 the undulation, and producing lateral resiliency, and this, re- 

 acting on the moving weight, redoubles its effects. In this way 

 a motion disagreeable to passengers, and dangerous to the 

 fabric, is unavoidably generated. The only question is, whether 

 it will be produced by a small or moderate weight on a ponderous 

 and extensive arch, and to this question it is not easy to give a 

 satisfactory reply. Much Avill depend on the roughness of the 

 road, and the rapidity and impetus of the moving body. There 

 cannot be much doubt that this sort of motion would not so 

 readily take place in the design submitted, because then every 

 weight as it passes along pulls (so to speak) directly to a fixed 

 point. 



These discussions apply to every description of catenarian 

 arch in which the rods or chains are placed over the road, but do 

 not affect such as Fig. 17, where they are all placed under the 

 road. In this case any number of rods may be adopted ; and 

 from there being no high tower, there can be no danger from 

 vibration. 



Compared with the construction now submitted, there are 

 other disadvantages attending a road suspended from a catenarian 

 arch, such as inequality of strain and pressure, difficulty of 

 attaching the rods at the abutments, &c. : but these and other 

 objections may be considered as local and peculiar ; while those 

 above enumerated apply to every possible mode of suspending 

 a road from an arch. 



