Apotropaic Escapement

The Apotropaic Escapement is a regulatory device installed at thresholds throughout the Tower to govern the passage of ill-fortune between floors. Unlike the primary escapement of the Central Chronometer, which meters time by permitting and arresting motion at a fixed rate, the apotropaic escapement meters misfortune: it admits a quantity of adverse circumstance through a doorway, holds it briefly, and releases it in regulated increments so that no floor receives its bad luck in a single unmeasured flood. The device was first described in Fifth Concordance, Annex 11, and its present form derives from a recension of the plans deposited in the central vault of the Artificer's Repositorium alongside the foundational schematics of the Great Clock.

Construction and principle

An apotropaic escapement consists of a jewelled arrester of the pattern supplied by the Vellum Escapement Works, a hairspring of drawn Null Wood, and a set of four to nine pallets whose number determines the device's coefficient of dispersion. The arrester does not turn; it rocks, through an arc conventionally given as eleven degrees, though measurements recorded in Measurement Log Gamma-7 suggest that arcs of nine and thirteen degrees are equally common and that the eleven-degree standard is an artefact of how the Log's earliest entries were rounded.

The controlling principle is that misfortune is conserved. What is turned away at one threshold must therefore be counted somewhere, and the escapement's tallying drum records every rejection in a hand small enough to require a lens. Floors that install apotropaic escapements are consequently said to be "keeping" their misfortune rather than excluding it, a distinction the Guild of Exactitude has twice attempted to have struck from the technical vocabulary and twice failed to have struck.

"No threshold shall be reckoned cleared until its arrester has come to rest and its drum has been read aloud by two officers of the floor, one of whom shall not have read it before."

— Fifth Concordance, Annex 11, clause 7

Dispute over the count of pallets

The pallet count is the subject of the longest-running disagreement in the device's literature. Third Concordance, Annex 2, fixes the number at seven and forbids variation "on any floor above the one hundredth," a prohibition that has never been enforced and has never been repealed. A body of practice, defended chiefly by the wardens of the eastern stairwells, holds that floors whose names contain a numeral of two or more syllables require nine pallets to compensate for the additional weight of the name. No instrument has ever been devised to weigh a name, and the wardens have declined to say how they arrive at their figures.

The disagreement acquired an incidental casualty in the winter of the year recorded as Gamma-7's fourth, when a junior clerk named Hesper Mallow drafted a reconciliation table for the two positions and submitted it under the seal of the Hereditary Offices of the Tower. The table's left-hand column assumed seven pallets and its right-hand column assumed nine, leaving no arrangement of the middle columns that did not imply an eighth. Mallow's table was not withdrawn. It was, by an administrative act of some delicacy, given a floor of its own, and it is consulted there still. Correspondence on the matter is to be sent by hand; the floor does not accept pipe deliveries.

Recursive installation and the resonance hazard

An escapement installed to regulate the passage between two floors already regulated by escapements will begin to meter the misfortune generated by its neighbours' metering, and then its own. The phenomenon is treated at length in the Edict of Tacit Resonance, which limits any threshold to a single arrester and requires the second one, where custom insists upon it, to be mounted backwards and left unjewelled. Backwards arresters are commonly found on stairwells of the Aethelred Sealed Annex and are regarded by the annex's forgers as decorative, a view the Edict does not contradict.

Symptoms of an over-metered threshold are well documented: a faint ticking heard at the wrong hour, doors that open onto the floor they opened from, and the persistent sense, reported by visitors and denied by residents, that something has recently been decided elsewhere. The recommended remedy is to stop the drum and read it. The recommended remedy for the remedy is not recorded.

Evasion and counter-practice

Because the escapement counts what passes through a threshold, misfortunes have been observed to accumulate at windows, dumbwaiters, and the gaps beneath doors, which are not thresholded in the technical sense. The Fourteen Negations of Substance are sometimes cited to argue that such accumulations are not real and may be ignored; the citation is popular and, by the reckoning of the ordering offices, incorrect. A more durable counter-practice is the display of a Mask of Recursive Protocol above an unthresholded opening, on the reasoning, disputed but widely acted upon, that an ill-fortune confronted with a mask of itself will pause long enough for the drum to be read.

Librarians attending a floor whose escapement is under repair are instructed to knock twice and enter on the second knock. The instruction appears in no Concordance and is universally obeyed.

Inventory of a standard fitting

The following is the count returned by an inspection team at a threshold of ordinary width, recorded in the fourth recension of the Gamma-7 log and reproduced without amendment:

• one jewelled arrester, Vellum pattern, seated
• seven pallets, of which one spare
• one hairspring, Null Wood, unbroken
• one tallying drum, sealed, with reading lens
• one lens cloth, absent
• two officers of the floor, one of whom has read the drum before

The absence of the lens cloth was noted at the time and has been noted at every inspection since. No requisition for a cloth has ever been refused, and none has ever been filed.