250 Note on Iron Tension Bridges. [March, 



of water-way, the stability or steadiness of the bridge, its durability, 

 as well as the quantity of metal required in its construction. It was to 

 the last only of these considerations to which my Memoir referred ; 

 and after enunciating the exact theoretical result to which I came, the 

 practical conclusion was stated thus : — 



" The economy of iron will be practically greatest in bridges where 

 the varieties* of tension are least. This tells, then, in a practical point 

 of view, against the Taper-Chain system, in the question Taper-Chain 

 versus Common Chain Bridge." 



But the Common Chain Bridge might be considered far less elegant, 

 or less convenient for the passage of boats under it, or less durable, or 

 less stable than the Taper-Chain Bridge for any thing my demonstra- 

 tion had proved. It was only on the Economy of Iron that I touched. 



I purposely confined myself to this one point, that I might avoid 

 being involved in controversy. And I thought I was safe from this, as 

 the proof of the proposition I enunciated is a mere piece of geometry ; 

 and the brief practical inference from it is so obvious that no one can 

 doubt its truth. 



At the time of my committing this demonstration to paper at the 

 request of another friend, f who also proposed that it should be printed* 

 I had begun to take up the question of the comparative stability or 

 steadiness of the two systems, especially when heavy weights pass along 

 the roadway, and its parts are successively pressed down ; and not si- 

 multaneously, as when the bridge is loaded uniformly from end to end, 

 by the greatest weight it can ever have to sustain. This view of the 

 subject might have been favourable to the Taper-Chain system. But, I 

 am sorry to say, that I found the subject so inviting, and therefore my 



* By " varieties," I did not mean changes of tension in the same bar as different 

 weights were placed on the bridge, but " varieties of tension'' in considering the 

 various parts of the bridge at any one instant. Thus, for example, in the common 

 Chain Bridge with vertical suspending rods, the strains of these rods, when the 

 bridge is uniformly loaded, are nearly the same, there is no variety : whereas in the 

 oblique suspending rods in the Taper-Chain, those near the centre are far more 

 strained than those near the ends of the bridge ; i. e. there is a greater variety of 

 tension. I have no reason for supposing that the expression I used has been mis- 

 understood. But we never lose any thing by adding to the perspicuity of our 

 language — especially in such subjects as the present. 



f Colonel Forbes. 



