Gunter's Scale

I am reading about it in Moore's "Practical Navigator" (1798) and in Andrew Mackay's "The Complete Navigator" (1807). Very clever device. I quote from Mackay

The ruler in general use in navigation, is that known by the name of "Gunter's Scale". The length of this scale is usually two feet, and about an inch and a half broad. One side of this scale contains lines for constructing geometrical figures; and the lines upon the other side are call "artificial", or "logarithmic" lines, being intended to resolve the questions in the several sailings, and to perform other mathematical operations.
Side 1:
  • Scales of equal parts
  • Chords, marked CHO
  • Rhumbs RHU
  • Sines SIN
  • Secants SEC
  • Tangents TAN
  • Semi-Tangents S.T.
  • Longitudes M.L.
Side 2:
  • Sine rhumbs S.R.
  • Tangent T.R.
  • Numbers NUM
  • Sines SIN
  • Versed sines V.S.
  • Tangents TAN
  • Meridioinal parts MER.
  • Equal Parts E.P.

The chapter goes on to describe how to contruct a gunter's scale mathematically. Very very clever device!

It seems that the Gunter's scale is a precursor to the slide rule. You use it by measuring off distances and values with a pair of dividers or compasses. The trig functions are defined in an equivalent, but different way, then they are usually defined today; as lengths of lines rather than ratios of lengths of lines.

Read about Edmund Gunter...

By Gordon Talge