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Chain of thought

Letters

I've enjoyed reading the recent series of letters in NCE on the use of chains in measurement.

The original surveyor's chain, known as the Gunter chain after its inventor, astronomer Edmund Gunter (1581-1626) was a reasonably accurate and useful linear measuring device.

The applicable measurement table was:

100 links = 4 rods, poles or perches = 1 chain 10 chains = 1 furlong 8 furlongs = 1 statute mile 10 square chains = 1 acre

None of this has anything to do with the 100ft chain that was an appallingly inaccurate, and difficult to use measuring device.

Half as heavy again as the Gunter chain, it was made of the same material but the slenderness ratio of the longer link made it much more prone to deformation and hence to inaccuracy. Beefing up the link was not an option because the additional weight would have made it 'un-throwable' for most of us.

Using such an inaccurate linear measure in conjunction with a theodolite, as one of your correspondents recalls, brings a wry smile - angular measurement to a few seconds of arc and linear measurement to a foot or two per 100ft.

My Gunter chain surveys were in conjunction with the miner's dial, of which even fewer of readers will have heard.

Tom Burke (M), mining consultant TomBurke@mctb. fsnet.co.uk

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