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Dring & Fage Spirit Excise

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`(7.1" Scale Proof scale, Alcohol Proof, c1800-50)`

This slide rule has been in the family for a long time but no-one knew what it was. It was researching this rule that provoked my interest in slide rules more generally.

The rule would originally have been one of a set of two rules , a 'Proof' and a 'Comparative' rule, which would have been used together with a hydrometer and thermometer to determine alcoholic strength and excise duty of spirits.

First the proof rule was used to calculate the strength from measurements of the alcohol solution, then using the proof obtained the comparative rule had two uses. The scales on one side could be used to determine the comparativevalues of strengths in shillings and pence, and on the other side to determine the comparative quantities of strengths in gallons. Duty could then be calculated, or strengths adjusted as required by concentration or dilution of the solution by the amounts described.

Dring & Fage were British instrument makers from 1790 to the 1970's, and practically had a monopoly in these types of Customs & Excise instruments.

This rule is a comparative rule made from ivory with brass straps, instead of the more usual boxwood, measuring 7 7/8 inch long, by 1 3/8 inch wide, by 3/16 inch thick.

The proof scale on both sides of the slide runs from 60 under proof to 70 over proof, with 'PROOF' marked at zero.

On one side the shilling scale, labelled "A", runs from just under 2 to 8 above the slide, and continues from 8 to 33 below the slide (labelled "A" again).

On the other side, the quantity scale, labelled "C", runs from 250 to 60 gallons, and continues from 60 to just under 15 gallons below the slide (labelled "C" again).

"DRING & FAGE MAKERS TOOLEY ST LONDON" is printed along one edge.

The address in the makers stamp and range of the shilling values dates the rule to the first half of the 19th century.

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