William Peddie

Correspondence with Peddie (Logarithmic Spiral)

D'Arcy Thompson had a lot of correspondence with the Scottish physicist and applied mathematician William Peddie (1861-1946) during the years before On Growth and Form. They were colleagues at Dundee University, with Peddie working there from 1907 as Professor of Physics. A large area of the their correspondence concerned spirals and how they relate to actual forms. In one of the key letters (ms45791), Peddie describes in detail the coiling of such a spiral and the mathematics behind it.

Thompson was particularly interested in the overlapping of spirals and Peddie believed this was to do with one coil partly caving in the other. He also had ideas on the way open and closed spirals were formed, using the rotation of one spiral by an angle Φ.

To see the full transcript of Peddie's letter on equiangular spirals click here.

darcy spiral1 — Peddie's sketch of a logarithmic spiral
(dashed line spiral caused by rotation of pole by φ).
Spiral becomes closed when φ is 2π. .

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