↑ Barrow calls the device a "vernier". See: John Barrow, Navigatio britannica: or a complete system of navigation … (London, England: W. and J. Mount and T. Page, 1750), pp. 140–142, especially page 142.
↑ Daumas, Maurice, Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers, Portman Books, London 1989 ISBN 978-0-7134-0727-3
