24 Design for a Bridge [Jan. 



but is merely suspended from their apex or ridge-piece, and as 

 all the rods he loose in grooves in the ridge-piece (see Fig. 3), it 

 follows that any resilience or vibratory motion of the road could 

 only affect the fulcrum by the friction of the rods (formed ot 

 roller links in that part) in sliding a few feet in the groove, when 

 a heavy weight (say 24 tons) passed along the bridge. This 

 sliding or motion supposed to take place in these grooves could 

 only take place in consequence of such immense weight being 

 in motion on the road, and would by no means be a general 

 thing. It would operate by rubbing on the cylinder in a down- 

 ward direction on the side in which the. weight was operating, 

 and in an upward direction on the opposite side. As the form 

 of the ridge-piece is a semicylinder, these motions would be 

 equal on each side, and would of course balance each other. 

 Thus balanced, they may be considered as resolved into a per- 

 pendicular pressure ; and taking the friction of metal on metal 

 (ungreased) at a quarter the pressure,* its amount is not 

 worth taking into calculation. It may be observed that, by 

 the mode in which the rods are distributed over the fulcrums, 

 the weight of no one part, including its strain and friction, 

 operates on any single point or column of the fulcrum, but always 

 On two separate and distant columns. Thus suppose two 

 waggons, or a weight of 25 tons, entering on the bridge at 

 either abutment, it would first bring the rods 13, 13, (see profile) 

 into a state of tension, then 12,12, and so on. When it arrived 

 under the first fulcrum, its weight would still be on two columns. 

 When in the centre, it would be equally divided between both 

 fulcrums, and the strain between both abutments ; and as it 

 proceeded from the centre to the shore, eveiy effect it produced 

 on the rods would resolve itself into a perpendicular pressure on 

 two distant parts of the fulcrum, and a strain on two parts of the 

 rock. The whole of this operation is widely different from the 

 effect of a weight on a road in part suspended by an inverted 

 arch, and in part resting on its fulcrums ; where - a continual 

 action and reaction on one, or on two points, must tend in some 

 degree to diminish the stability and safety of the fabric. 



III. As to the Resistance to Resiliency, or undulating Motion. 



In all the chain and. rope bridges which have hitherto been 

 erected (at least as far as I can learn), this motion has occa-r 

 sioned a prejudice against them, both on account of its incom- 

 modiousness, and the injurious effects of the friction produced 

 on the edifice. -\ 



There are only two ways in which this motion is to be resisted : 

 by the ponderosity of the materials, or by construction. The 

 first is a clumsy method, not likely to answer all the ends in. 



* Dr. Young, Lect. XIIT. 



+ Sir Howard Douglass on Military Bridges. 



G 



