On the night of October 16, 1834, workers burning two cartloads of obsolete wooden tally sticks in furnaces under the House of Lords set the Palace of Westminster on fire. The blaze took the Lords' chamber, the Commons' library, and most of its procedural records, along with a single brass rod kept in the custody of the Clerk of the House of Commons. An Act of Parliament in 1824 had made that rod the Imperial Standard Yard: the one object every chain, scale, and surveyor's rule in the kingdom ultimately answered to.
The 1824 Act had anticipated losing it. If the rod were ever lost, destroyed, or defaced, the law instructed, the yard could be recovered from a pendulum beating seconds of mean time in the latitude of London, in a vacuum at sea level, and the pound from a cubic inch of water: no artifact, just physics. A royal commission, working under the Astronomer Royal, George Biddell Airy, spent years weighing how to carry that out after the fire, and concluded the method could not be trusted: research already on record showed doubtful and erroneous corrections in the very pendulum experiments it depended on. The 1855 Act that finally settled the matter says so plainly, in its own recital.
What rebuilt the yard instead was redundancy nobody had designed for the purpose. Other bars had been measured against the Standard before the fire for unrelated reasons: Sir George Shuckburgh's own scale, the Royal Society's copy of the yard, which Henry Kater had compared microscopically against the Standard in 1831, and two bars the Ordnance Survey kept for mapping work. Richard Sheepshanks, who took over the work in 1844 after Francis Baily died, compared a new provisional bar against each of those survivors. The different routes gave slightly different answers for the lost yard's exact length, agreeing with each other only to a few ten-thousandths of an inch, so Sheepshanks took a figure close to their average as near enough to the truth to build new bars from. The fire-damaged rod itself, recovered and examined, could not help: the commission's own report says it was not available for restoring the lost standard's value.
Parliament legalized the reconstruction in 1855, repealed the pendulum clause outright, and declined to trust the result to one object again. The restored Standard Yard went into the Exchequer, and four accurate copies, each cast and calibrated separately, were deliberately scattered: one to the Royal Mint, one to the Royal Society, one to the Royal Observatory at Greenwich, and one sealed into the stone of the rebuilt Parliament itself. The new law asked for no further elegance. If the standard were ever lost again, it said, the Treasury could restore it by reference to whichever of the copies still existed.
A fact kept in exactly one place is a fact one accident away from disappearing. The 1824 Act had tried to guard against that with a formula, a repeatable measurement standing in for a kept object, and once anyone actually checked the evidence behind it, the formula did not hold; what actually worked was several ordinary measurements that happened to agree. The measure that mattered, by 1855, was not the one true original. It was how many independent places still agreed on what the original had said.