History of Science and Technology/bridge

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Haverhill 1820
Haverhill 1820  
I am attaching an image of a bridge in Haverhill, Massachusetts, in 1820. It looks "modular"; somehow, they must have been able to move the middle sections to allow sailboats to pass. It looks as though there is a huge, sharp "dip" where the segments join, which would have made it impassible for wagons. Do you know how these bridges worked, and whether there was something spanning each section to cover the "dip"?

Thanks,
Steve

Answer
Hi Steve--I'm guessing this bridge no longer exists, so you can't get a good look at it now....  I can't tell you for sure, but here are my guesses.  From the configuration in the image, it might be a swing bridge:

http://en.wikipedia.org/wiki/Swing_bridge

Neither this webpage nor anything else I can find immediately online seem to say anything about the history or origins of the swing bridge; the one near me dates from the 1870s, but I don't know how much older than that this technology is.  It's also possible that this is just a standard non-moving three span bridge; in the image I see a boat on the 'big' (right) side of the river, but no boats or places to moor boats on the other side.

I'm inclined to think the dips in the image are either artistic exaggeration or the artist not understanding what he/she is looking at (e.g. visualising dark places on the roadway, which could be seals between either moving or nonmoving segments of the bridge, as shading of convex surfaces); I can't imagine what purpose or requirement such dips would serve.  It's certainly common to see such artistic misinterpretations in drawings of of engineering works, so that's not a completely off the wall explanation.

I hope this helps, if only a bit--Carolyn  

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Carolyn Dougherty

Expertise

I can probably answer most general questions about the history of science, technology and engineering from ancient to current; if I don't know a specific answer I can probably refer a questioner to an appropriate source. I have done original research in the history of computing and in British science and engineering in the 17th, 18th and early 19th centuries.

Experience

I am a PhD student at the University of York, writing about the carrying trade in the 18th century; my previous work at the university includes the early history of plate railways. I have taught courses in the history of science and engineering at York and other universities, and have presented several papers on various subjects in this field at academic conferences. I am also a practicing civil engineer.

Education/Credentials
BA--Berkeley, MSc--Berkeley, MA--York, currently working on a PhD at York.

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