120 On the Comtractiori of Suspension Bridges [Jan". 



valuable correspondents of the Madras Journal will favor us with their 

 opinions on the subject. I say cheap, because of all low priced sub- 

 stances that may be pronounced durable, iron wire has four or five 

 times greater strength in proportion to its sectional area. Thus bars 

 of welded iron are allowed a supporting power of nine tons only 

 per square inch of sectional area, while iron wire is admitted to have 

 an average strength of 38| tons ! (the experiments on the continent 

 give 40 and 47 tons per square inch) ; and the smaller wires are stronger 

 in proportion than the (farmer. A single one-tenth inch wire will bear 

 688| pounds, that is, such a weight is always safely allowed as its ulti- 

 mate strength. Now 48 wires -^-^ inch diameter give a sectional area 

 of 0.376992 inches, and consequently will bear 141 tons; nearly four 

 times the greatest load that can be brought on them in any bridge of 

 twelve feet road-v;ay, with supports ten feet apart; and these forty-eight 

 wires 114 feet long T the calculations having been made for a bridge 

 1 12 feet span), will weigh less than 1401bs. ; being only 139.86 pounds : 

 and the saving of wire alone in the main chains arising from this de- 

 creased load in comparison with an ordinary platform for the same 

 span (although the wire for the new one is taken into account) is so 

 great as to reduce the expense in this item alone, nearly 1000 rupees. 

 At the same time by thus raising the road-way bodily above high wa- 

 ter mark, the basements of the suspending piers may be reduced in 

 height, and various other important savings will occur by the lessening 

 of Carpenters' work, iron work, &c. &c. To prove that I am not alto- 

 gether visionary in my views, I annex a copy of a memorandum taken 

 by a friend from the 29th No. vol. 15 of the Quarterly Journal of 

 Science, 



" Economical Bridge. — A bridge of suspension, or rather tension, has 

 " been constructed not long since by M. M. Seguin, near Annonay, 

 " department de I'Ardeche, after the model of those constructed by 

 " the indigenous inhabitants of America. The following description is 

 " from Monsieur Pictet. 



" At the place where it is constructed, the river over which it passes 

 " is confined by rocks which have furnished strong points of attach- 



ment for the bridge. A band composed of 8 iron wires, each the ~ of 

 " an inch in diameter is attached by its extremity to an iron bolt fixed 

 " in the rock ; it then crosses the river at a height of ten feet above it, 

 " and on the opposite side passes round a horizontal pulley three inches 

 " in diameter, also made fast to a rock. The band returns parallel to its 



