183 a] 



Observations on Suspension Bridges. 



329 



The current runs through the passage in the Paumban sand Ijank 

 with the rapidity of a mill race, and I conjecture would impart motion 

 to the largest description of water wheel, and most effectually sur' 

 mount any resistance opposed to its revolution by the sand and silt, it 

 might come in contact with m the bed of the channel, I therefore re- 

 commend that three rafts, constructed on country boats, be moored 

 across the channel, one on either hand, and the third midway, and that 

 scaffoldings be erected upon them of sufficient strength to sustain two 

 water wheels armed with spades or shovel leaves, which, in their revolu- 

 tion, will dig into the silt at the bottom and bring it up with them ' 

 the contents of these vessels in the shape of sand, silt, and rubbish, on 

 being discharged into lighters moored behind the wheel, will in the 

 descent from one leaf to the other, after passing the vertex, materially 

 aid the rotatory motion of the wheel, which may be further increased, 

 if necessary, by manual labour, on the rafts, applied to a double wynch 

 bent in the prolongation of the iron axis of the wheel : the flat bottom 

 of the shovels, which are fixed at a trifling inclination to the spokes, and in 

 the direction of the current, offer that measure of resistance to the stream 

 requisite to convey motion to the wheel independent of the foregoing 

 additional powers. 



The apparatus may be otherwise disposed, by a single wheel being let 

 into tlie deck of the raft, where it will play between the boats that sup- 

 port the platform ; it thus becomes more locomotive, and offers no ob- 

 stacle to the navigation of the passage by the country vessels during 

 w"orking hours." 

 Palamcottah, August 23, 1838. 



V. — Observations on Suspension Bridges, ivith a Plan. — By Lieut. C. C, 

 Johnston, Madras Engineers. 



Suspension Bridges are particularly adapted to the traffic in this country, 

 as they are never likely to be strained with very heavy loads. The expence 

 of their construction would not exceed half that of masonry bridges, and a 

 very great advantage to be derived from th^m is, that they may be erected 

 at any time, without regard to the quantity of water in the river. This re- 

 mark applies only to bridges averaging 100 feet span, for any thing be. 

 yond that, it would be advisable to have one pier (or more as the case 

 may be) in the bed of the river. The following calculation is made for 



