PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 895 



Tubular Girdeks. 



Noticino- the first system, we have before us a magnificent type 

 in the Menai and Conway bridges of Stevenson and Fairbairn. 

 These works are probably well known to you all, as well as the 

 story of their inception and completion. You can recall the vast 

 outlay in brains, time and money required to prove the feasibility 

 of the idea, and although the adaptation of the tubular girder for 

 great spans, and its level roadway, allewing gi-eat height, its ugli- 

 ness and cost, in connection with the advanced state of the science 

 of bridge construction, will allways militate against its use here- 

 after. 



The total length of the Britannia bridge is 1,511 feet, in two 

 spans of 460, and two of 230 feet in the clear. The two tubes 

 (one for each line of rails) emplo3'^ed in their construction nearly 

 9,500 tons of wrought iron, 1,015 tons of cast iron, and 165 tons 

 of permanent way. These two tubes are composed of 186,000 

 separate pieces of ir.on pierced by 7,000,000 holes, and united by 

 over 2,000,000 rivets. They contain 83 miles of angle iron, and 

 their total weight is 10,540 tons. The total cost of the bridge 

 was about $3,000,000, nearly $2,000 per lineal foot. At the 

 present time, in this country, we may safely say that $6,000 per 

 lineal foot would represent the price of its reproduction. From 

 these few figures I think that you will see very clearly that the 

 question of cost puts the further consideration of this system aside. 

 So much for the tubular system. 



Trussed Girders. 



We now come to trussed girders, wdiich are an outgrowth of the 

 problem always presented to the American engineer, and that is 

 to accomplish the greatest results with the least expenditure of 

 time and money, stand in the names of the various patentees, and 

 we have the original " Whipple," the parent of the trussed cast 

 and wrought iron girders, the " Murphy Whipple " and the 

 "Linville Whipple," these two last varying from the parent 

 bridge in details merely. Then again there are the Fink and 

 Bollman trusses, the Lattice girder, and what is known in England 

 as the Warren triangular jrirder. There are a few others, not so 

 well kuoAvn. as the "Post" truss, a cross between the triangular 

 and Whipple bridges. The efforts of the designers, in all these 

 trusses, is to use that material which, with the least expense, is 

 best able to resist the particular strain that comes upon it, and to 



