Britain taxed spirits by strength once one instrument's reading became unarguable

British excise duty on spirits was levied per gallon of 'proof' strength, but proof itself had no single agreed measurement: distillers and revenue officers each favored different hydrometers — floating instruments that read alcohol content by density — and competing designs gave different readings for the same liquid. That gap between instruments meant every tax assessment could be, and often was, contested in court, costing the Crown revenue to disputes and giving distillers room to argue down their bill by pointing to whichever device read the strength lower.

Clarke's hydrometer, the instrument nominally in official use, was known to be inaccurate and easily manipulated, but simply mandating stricter enforcement of it wouldn't fix a measurement that was unreliable at its core. The Crown needed not tighter policing of a flawed instrument but one instrument accurate and standardized enough that its reading could not be reasonably disputed.

Made one hydrometer design the sole legal measure of spirit strength, so there was no second reading to appeal to and proof became whatever the Sikes instrument said.

Once Parliament named a single instrument the sole legal measure, there was no longer a second reading to appeal to — a distiller could still dispute the tax rate, but not what the proof was, because

This only works when a government or authority actually has the power to mandate one instrument by law; in voluntary markets without that enforcement power, competing measuring standards can persist i

When a tax, contract or payout depends on a measurement that multiple parties can each read differently, the fix isn't stricter enforcement of the disputed instrument — it's making one specific instru

The Sikes hydrometer became the sole legal standard for spirits duty in Britain, ending disputes, and stayed official until 1980. The 'sikes' scale it created stayed embedded in British and Commonwealth alcohol law for over 160 years, and its principle — that a contested tax base needs one legally fixed measuring instrument, not better enforcement of a disputed one — became a template for excise systems elsewhere.