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Tuesday, August 16, 2011

Newton's Mathematical Bridge

Punting on the Cam, approaching the Mathematical Bridge.
Image courtesy of Alan Fleming/Wikipedia, 2005.

The University of Cambridge, or Cambridge University as it is informally known, was founded by scholars in 1209, who left Oxford after a dispute with the townsfolk.  Graduates have won 61 Nobel prizes, more than any university in the world.  The university is composed of 31 colleges which are self-governing and independent, each having their own property and income.

Queens' College, Cambridge.  It is called the Queens' College (plural) because it was
founded in 1448 by Margaret of Anjou (Queen of Henry VI) and refounded in 1465 by
Elizabeth Woodville (Queen of Edward IV).  The full name is The Queen's College of
St. Margaret and St. Bernard, but it is commonly called the Queens' College.

Among the many esteemed alumni is Sir Isaac Newton.  There is a story about a wooden footbridge that links two parts of Queens' College over the River Cam. Legend has it that Newton designed and built the bridge without using any bolts or nails.  Further rumor is that at one point students and Fellows disassembled the bridge and tried to rebuild it, but were unable to do so without any fastenings - nails, bolts, screws.

Oldest known photo of the bridge, ca. 1853.  Taken by Thomas Craddack
while he was an undergrad at Queens'.  Image courtesy the Queens' website.

Despite facts to the contrary, the story persists, especially as a favorite to tell tourists.  The bridge is one of the most photographed scenes in Cambridge.  But the most blatant problem with the story is that Newton died in 1727, and the original bridge was built in 1749.

The original bridge in 1865.  Note the railings are leaning inward, and the
timbers are showing wear.  The repair only lasted 40 years.  Image from here.

The original bridge was built by James Essex the Younger to the design of William Etheridge.  It was repaired in 1866, and rebuilt using the same design in 1905, with one change.  The original bridge was stepped.  The rebuild was sloped to accommodate trolleys (this now makes it wheelchair accessible).  The college still has an old model of the bridge (with screws at the joints), which is presumed to be Etheridge's model from 1748.

Image showing sloped ramp, courtesy of www.bluffton.edu.

In the 1905 rebuild, teak was used instead of oak.  The work was done by local builder William Sindall.  The joints were fastened by nuts and bolts passing right through the joints, with the bolt  heads on the inside, thus visible. Since the original bridge had the fastenings visible only on the outside, that may have given rise to the rumors that none were used.

Easy to see why it is a popular subject to photograph.  Image courtesy of www.kaplan.com.

The timbers are arranged in a series of tangents with radial members to tie them together and triangulate the structure so that it is self-supporting. A tangent and radial trussing, technically, it is an efficient use of timber and has also been used for the supporting arches when building stone bridges.

Photo with tangential timbers highlighted.
Amazing that this rumor has lasted so long at the second oldest university in the English-speaking world, and one with an impeccable reputation.  But that says more about humans than it does the institution.  ;-)

Unless otherwise noted, all images courtesy of Wikipedia.

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