At the northern end of Trafalgar Square, facing the National Gallery, and largely ignored by tourists is a brass plaque with inset the official Imperial units of measurement. The inch, foot and yard are defined at 62 degrees Fahrenheit, with other plaques in the square showing the chain (66 feet) and 100 feet. Now, because of the construction work that has recently gone on to make pedestrian access easier between Trafalgar Square and the National Gallery, the plaque has been moved slightly east of the original position it has been in since 1876. I am not sure if the other two plaques have also been moved to maintain their original purpose.
How did the plaques get to be located in Trafalgar Square? Here lies a story of technological modernization bumping into administrative incompetence. The Exchequer of the British government used to use elmwood tally sticks to record its transactions. These were designed to be in two parts so that both parties to any transaction held half the evidence, but only when both were brought together could any new data be added. However, these were cumbersome to store and as early as 1724 Treasury officials demanded they stop being used, upgrading to the then high technology of pen and paper. But it was not until 1782 that the Exchequer ceased using them and 1826 that all government departments finally stopped using them. Their popularity partly due to the high level of illiteracy, even amongst the lower levels of government functionaries.
So we finally come to the year 1834, when a decision had to made of what to do with the huge stock of old and useless tally sticks. As Charles Dickens comments,"The sticks were housed in Westminster, and it would naturally occur to any intelligent person that nothing could be easier than to allow them to be carried away for firewood by the miserable people who lived in that neighbourhood." However, some paranoid officials thought it unwise to let the sticks out into the public just in case someone were able to piece together the tallies and thereby be privy to confidential information. So, in the name of national security the horde of tally sticks was scheduled to be burnt in the House of Lords furnace.
Disaster quickly followed, in the words of Dickens again,"The stove, overgorged with these preposterous sticks, set fire to the panelling; the panelling set fire to the House of Commons; the two houses were reduced to ashes..." What is relevant to our story is not so much the burning of Westminster but the destruction of all the standards of the Imperial units of measurement. A Standards Commission was hastily set up, chaired by Sir George Airy, with the task of reconstructing the measurements.
So as not to repeat this unfortunate fiasco, the Standards Department of the Board of Trade further decided to duplicate the standards and located the standards of length in three places: in Guildhall; the Greenwich Royal Observatory; and in Trafalgar Square.