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Calculating rule, duplex, boxwood, top edge is a 24 inch ruler (reading right to left), bottom edge bevelled. Scales are engraved into wood in colour black.

The scales on this rule are specifically designed for navigation.

Scales:

Obverse:

Left half:

24 inch ruler (reading right to left),

a grid of lines at ½ inch spacing with nomographs at either end.

Right half:

LEA - Leagues

RUM - Rhumb

M.L - Miles of Longitude

CHO - Chord - in degrees of Latitude

SIN - Sines

TAN - Tangents

Reverse:

S.R - Sines of Rhumbs

T.R - Tangents of Rhumbs

NUM - Line of Numbers -

SIN - the Sine of the angle on this line is read off on the NUM scale

V.S - Versed Sines

TAN - the Tangent of the angle on this line is read off on the NUM scale

MER - Meridian line - the distance along a meridian line of degrees of latitude.

E.P - Line of Equal Parts

Name on white paper sticker on rule on reverse, in upper left corner, handwritten in black ' Bromely / 1983-53 '.

Maker unkown.

The development of the slide rule went through several stages. The first significant device was Gunter's Line of Proportion (also known as the Gunter's Rule or Gunter's Scale) invented by the English astronomer and mathematician Edmund Gunter in 1624. He engraved a two foot long (60cm) straight-edge rule with lines marking the numbers placed at distances proportional to the logarithms of the numbers.

By using a Gunter's Rule multiplication or division can be carried out simply through the addition or subtraction of distances on the rule using a pair of pointers or compasses. In order to multiply two numbers, one pointer of the compass is set down on the scale at the index marker (the number 1) on the line of Numbers and opened so that the other pointer of the compass extends to the multiplicand (the number to be multiplied). The compass is then re-placed with one leg at the multiplier, and its other leg now points to the product of the two numbers.

The earliest Gunter's Rules had few scales on them, simply the line of numbers in logarithmic proportion, a line of squares and a line of cubes of the numbers plus the trigonometric scales which are useful for gunnery and navigation. [Hopp, p.9] They became the primary tool for navigation over the 18th century and were considered accurate enough for them to be still in use for navigation in the mid-19th century. [Cajori, p.21]

Refs:

Peter Hopp, "Slide Rules, their History, Models and Makers", Mendham, New Jersey: Astragal Press, 1999.

Florian Cajori, "A history of the logarithmic slide rule and allied instruments", Mendham, N.J.: Astragal Press, c1994.

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