1838.] 



Observations on Smjxnslcn Bridrjeft. 



K'blms ^'^-t/^^l = at 1 ™pee per e„l.ic foot, 128-. 

 Allowing 50 cubic feet for parapet do. do. 50 



2 abutments each Ak x ^\ x lOi = 245tV x 4 = 9811 



4 blocks of masonry for 

 ends of chains each 



31^ 63 X 4 = 252 



Total cubic feet 1233| at-k ^g5_?_ 

 Ebot j ' 



1 1 anna per cubic foot 

 Excavating foundation 20 x 10 x 5 ^ lOOOc. ft.— at 3 pice pr. f.— 15| 



-\ The cost of workmanship for the chains will vary 



Making chains do. f '^ery considerably, and will be very little, particu- 



Do. planking do. ? lady if each chain is 15 or 16 feet long. The data. 



} for masonry also vary in every part of the country. 



The trials that I have had the opportunity of making on suspension 

 bridges, are of course very limited, but at the same time quite sufiicient 

 to convince me of their practicability and very great value in this coun- 

 try. In a small bridge, constructed on the principle recommended by 

 Captain G. Underwood of the Engineers, (see No. 18 of the 

 Madras Journal) of 30 feet span, and 4 feet road- way, the main chains 

 consisted each of three wires of 1-lOth inch diameter, and giving a sec- 

 tional area of .0471 square inches, and allowing ten tons per square inch 

 forihe bearing weight gives .471 tons == 1055.57-lbs. this bridge I load- 

 ed with planking 30' x 4' x 1-6 = 20 cubic feet at 40-lbs. per cubic foot = 

 800-lbs., and then walked over it several times, the vibration was of 

 course very great, from the bridge being so light, but it was perfectly 

 strong. I afterwards loaded it to considerably upv/ards of half a ton be- 

 fore its giving way. 



I then constructed a bridge of the same dimensions, and supported the 

 platform on chains, each of one iron bar of | in. diameter, giving a sec- 

 tional area of .0981 square inches, and allowing 9 tons per square inch 

 for the bearing weight, gives .8829 tons or 1977.69-lbs. This was also load- 

 ed with the same weight, of planking as the last, viz. 800-lbs. and co- 

 vered with earth to 30 x 4 ^ 1-12 10 cubic feet at UO-lbs. = lyOO-lbs. 

 making a total of 2500-lbs. which is considerably above the bearing 

 strain allowed: this weight the bridge has borne for many weeks, and is 

 still standing in perfect order. Being obliged to leave the place v^diere I 

 was making these trials, I am unable to say anything farther on the sub- 

 ject, but trtff^t that these few observations may assist in promoting the 

 construction of such very valuable, practicable, and cheap bridges, so 

 p.--..' i'-vilv.-ly ad'^pted to this country. 



