Great Tuning Fork

The Great Tuning Fork is the collective designation for the eleven principal resonance instruments of the Tower's upper archive, of which nine remain serviceable. The largest and most frequently cited is the Great Tuning Fork of Epsilon, a 47-metre nickel-alloy rod suspended from the ceiling of Floor 11399 above the resonant chamber of Floor 11398. The remaining ten are distributed across floors 8,412; 13,006; 22,190; 22,191; 41,700; 60,005; 60,006; 88,441; and a tenth whose floor number is disputed between the Office of Vertical Records and the Bureau of Floor Registry. An eleventh fork, catalogued in the founding inventories, is listed as not presently locatable and has been so listed since 1746.

The forks are not tuned to one another and are not intended to be. Each establishes the fundamental resonance of the floor on which it hangs, and floors without a Great Tuning Fork are described in the technical literature as "borrowed," depending on the fork nearest them in vertical terms whatever the intervening floors may be. The arrangement is set out in Eleventh Concordance, Annex 2, which also prohibits, in a single unnumbered sentence, the carrying of open flame past a suspended fork above the seventh gallery.

Construction and suspension

Each of the eleven forks is struck from a single billet of nickel alloy, hardened by the Slow Anneal and struck under the supervision of two witnesses whose names are entered on the billet itself prior to forging and are consequently lost. Suspension is by braided cord of the Stairwell Guild pattern, replaced every twenty-two years by the same crew that maintains the vertical rails. The cord is checked each morning by a sexton who records the hour of the check and the weather at the check, though no use has ever been found for the weather.

Ringing is permitted on three occasions: the Attestation of Local Permeability, the opening of a new spiral, and the settling of a question put under Concordance of the Ringable. The forks have been struck in error four times in the present century, twice by the same visitor, and the Office of Acoustic Integrity has since 1903 required visitors to the chamber floors to declare their hearing before entering.

The doctrine of the eleven and the doctrine of the one

Scholarly dispute concerns whether there are eleven Great Tuning Forks or only one, encountered repeatedly at different heights by different expeditions. The position of the Vibrationist School is that the forks are eleven, that they are physically distinct, and that their failure to agree with one another is evidence of the School's doctrine of Resonant Tuning. The position of the Office of Vertical Records, argued at length in its 1889 memorandum and never withdrawn, is that there is a single Great Tuning Fork and that the Tower's floors rotate about it in a manner that makes the fork appear to hang in different places. The Office has declined to say which way the floors rotate.

"The rod is one. That it is seen at Epsilon, at 22,190 and at 88,441 is a matter of where the observer is standing when the bell is rung, not of how many rods there are."

— Office of Vertical Records, Memorandum on the Multiplicity of the Great Fork, 1889

The counter-argument, advanced by Clerk Amandine Vesk of the Office of Acoustic Integrity in 1902, notes that the fork at 22,191 hangs eleven metres from the fork at 22,190 and that the two do not resonate together, which would be difficult if they were the same rod. Vesk's note was filed under correspondence and answered in 1911 under other correspondence. It is regarded as the strongest objection on record.

Relation to the standard patterns

The Great Tuning Forks are not working references. Calibration at every level of the Tower is carried out against the Standard Tuning Fork, Pattern Gamma, and personnel on walking inspection carry Tuning Fork, Pattern 7. The Great forks function rather as an anchor: permeability measurements taken in a spiral are referred upward until they reach the nearest Great Tuning Fork, and are then referred no further. The Great Western Spiral returns its measurements to Epsilon; the Great Northern Spiral returns its own to the fork at 60,005, by an arrangement older than any surviving document and now maintained by habit alone.

The forks are also the last resort of the acoustic courts. A candidate Definite Silence is mapped with the Tuning Fork of the Seventh Silence, but a silence already attested may be appealed by ringing the Great Tuning Fork and observing whether the silence persists. If it persists, the appeal fails. The procedure is used perhaps twice a decade and is thought by most Librarians to settle nothing.

Incidents

During the winter of 1911, when the upper archive was closed for the great recension, the fork at 22,190 was found hung with a linen sack by persons unknown. The sack was removed without being opened. An inquiry under Concordance of the Ringable, Annex 4, established that no one had been admitted to the floor for nine days, that the sack was of a weave not made in the upper archive, and that the matter was to be regarded as closed. It was so regarded.

A second incident, in 1957, involved the fork at 88,441, which was reported to have been heard faintly for the space of a minute by staff on floor 88,439, two floors below and separated from the fork by a solid vault. The staff on 88,440 heard nothing. The report was forwarded to the Office of Acoustic Integrity, which returned it endorsed audible above only, and no further investigation followed.

Maintenance

The forks are cleaned annually with a dry cloth by a sexton on a ladder, the lower prongs first. No water, oil, or resin is permitted on the forks, and the polishing of a Great Tuning Fork is treated in the disciplinary records as a form of tampering with a floor's fundamental frequency. The annual cleaning is the only occasion on which a sexton is allowed above the seventh gallery without a witness present, a rule introduced after the sack.