Sikes's Hydrometer Scale
The scale that decided how much tax your whisky owed.
Origin
Bartholomew Sikes’s instrument was made the legal standard for British excise by Act of Parliament in 1816, and remained so until 1980.
The catch
Proof spirit was not the strongest spirit but a specific legal reference point: 57.15 per cent alcohol by volume, being the strength at which a spirit-soaked gunpowder charge would still ignite. Everything was taxed relative to that, so spirits were described as so many degrees over or under proof rather than by their actual strength.
The bottom of the scale: 100 under proof.
A typical bottled spirit strength today.
A strong bottling, still comfortably below proof.
The legal reference point, from which duty was calculated. Navy strength gin is still bottled at about this strength.
Cask strength whisky, as it comes from the barrel.
Absolute alcohol. The top of the scale.
Before Sikes, excise officers tested spirits by soaking gunpowder in them and applying a flame: if the powder still burned, the spirit was above proof and taxed accordingly. Sikes replaced the flame with a brass hydrometer and a set of temperature correction tables, which was safer, more repeatable and considerably less entertaining. Britain moved to percentage alcohol by volume in 1980, aligning with the European system, but the old reference point survives on the shelf: navy strength gin is still bottled at about 57 per cent, the strength at which the gunpowder would have lit.