1821–1884. Joseph Long: Alcohol proof index slide rule.

This slide rule is labelled Joseph Long, Maker, 20 Little Tower St, London. Long operated his firm at this address from 1821–1846, but the firm continued to operate under his name at the same address until 1884.1

The instrument was designed for excise officers, and was used for calculations relating to the strength of spirits. It is made of bone (probably ox bone) and is 9 inches in length with two juxtaposed scales held apart by brass clamps at each end, between which a slide moves on wooden runners. There is no cursor.

The front face of the Rule is marked A B and shows the comparative values of spirits of different strengths. On this side:

  • the line A is a scale of shillings per gallon, from six to thirty shillings.
  • the line B is a scale from one shilling and sixpence to eight shillings per gallon.

The back face of the Rule is marked CCC and shows the amount of water required to reduce a given volume of liquid from one strength of spirit to another.

  • the top C scale runs from 60 to 300 gallons
  • the bottom C scale runs from 20 to 100 gallons

The sliding centre rule has identical scales on back and front which show the different strengths of spirit from 70 degrees under Proof to 70 degrees over Proof . Here the figures to the left of the word “Proof” are measures of under Proof, and to the right are measures of over Proof.2

Examples:

  • Front face (AB): if the amount charged on proof is 15 shillings per gallon then that charged on 10 degrees overproof is 16 shillings and sixpence per gallon; if the excise on proof is 4 shillings per gallon then 10 degrees overproof is 4 shillings and sixpence and one farthing per gallon.
  • Back face (CCC): if 150 gallons is at proof, then to reduce it to 10 degrees underproof dilute it to 166 gallons by adding 16 gallons of water.
 

1 See Truman G. Blocker, Jr. History of Medicine Collections, British Firms: Long Joseph, viewed 18 June 2012; for a reference in 1872 see The London Gazette, 26 April 1872, p. 2079, item 1118, viewed 24 June 2012. . See also Webster Signature Database, viewed 24 June 2012. (↑)

2 For a more detailed explanation which is in any case consistent with this see “Directions for the Use of The Comparative Rule”, Robert Manley’s Slide Rule Site, viewed 3 July 2012 (↑)

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