Stilled Air

Stilled Air is the condition obtained when a body of air within the Tower is brought to rest and held there, neither circulating nor settling nor exchanging with the air around it. It is a state, not a substance; it has no colour, no weight beyond that of ordinary air, and no smell, although the Guild of Still Air maintains that it has a slight one and has petitioned four times for funds to characterise it. Stilled Air is distinguished from dead air, with which it is frequently confused, by the requirement that it be attended: a volume left to itself is merely stagnant, whereas a volume under one of the standing stilling protocols is Stilled, and is recorded as such.

The condition is chiefly associated with the Edict of Still Air and with the extensive apparatus of Atmospheric Calibration that administers it. It should not be confused with Stilled Light, with which it shares a name and no property whatsoever; the two offices have spent nine hundred Cycles declining to be consolidated.

Definitions and the counting problem

The Edict of Still Air, Clause 2, defines the condition as air "at rest in all three directions and in the fourth, where the fourth is required." The qualifying clause is generally read as a concession to the Great Northern Air Shafts, where vertical draughts make the ordinary three-direction test unworkable and where the fourth direction has never been satisfactorily described, even by the shaft's own wardens.

The practical difficulty is that stilling is measured by absence. A volume of Stilled Air is known to be Stilled principally because nothing has happened in it. The Circulation Division therefore maintains a small number of reference volumes — six at last count, held in the Ventilation Spirals and checked against one another — in which nothing is permitted to happen at all. Two have been lost to condensation. One is reported to have recorded an event in the 9,904th Cycle, which the Division has classified as instrument error and does not discuss.

The Stilled Air incident of Floor 47

Stilled Air is best known for having failed. During the 14,142nd Cycle a volume of Stilled Air on Floor 47 was found to have moved, and the resulting disruption of the floor's pressure balance is recorded as the 'Stilled Air' incident. The response was the Motet of Dispersal, a seven-hour chorus performed at the Lambda Shaft nexus by a Ventilation Chorus drawn from the Curators of the Still Air and the Guild of Still Air jointly, a collaboration that has not been repeated.

Contemporary accounts are unusually consistent on the outcome and unusually silent on the cause. The post-incident register of Floor 47 records only that "the volume was found to have been still, and then to have been less so" (Floor 47, Incident Register, entry for the fourteenth of Vell). The Unidirectional Flow Doctrine was cited in the margin without explanation, and the citation has been copied forward into every subsequent recension of the incident, including the abridged one.

Liturgical and administrative uses

A Stilled volume is a requirement in several procedures where a reading must not be disturbed. The Cartographers' Spire stills the air of its upper galleries before a survey plate is exposed, a practice defended in its Standing Orders on the ground that a moving particle produces a moved mark and a moved mark is indistinguishable from a discovery. The Parallax Halls retain a smaller stilling closet for the neutral checks conducted with Stilled Silver, although the closet has not been operative since the Waning of the West Stack reduced the supply pressure below the threshold named in the closet's own annex.

"No plate shall be exposed in air that has not been stilled and certified, and no certification shall be accepted on the same day it is issued, the certifying officer being presumed to have been in the room."

— Cartographers' Spire, Standing Orders, Article 11(c)

The presumption is not a slur on the officers. It reflects an old and unresolved property of the condition: the act of certifying a volume of Stilled Air requires an observer to stand in it, and an observer is, strictly, a disturbance. The Ventilation System's own manual concedes the point and offers no remedy beyond the instruction that the certifying officer should "be brief."

Controversy: the Fifth against the Ninth

Scholarly disagreement turns on whether Stilled Air is a condition or a collection. The Fifth Concordance, Annex 2, holds that stilling is a relation between the air and the apparatus, and that the same air may be Stilled in one gallery and unstilled in the next without any change in the air itself. The Ninth Concordance, Annex 6, holds the opposite: that stilling is a property of the air, is carried with it, and persists for a short interval after the apparatus is removed — an interval the Ninth declines to name, describing it only as "brief, and not to be relied upon."

The dispute is not academic. Foundation Charter Clause 17 treats the two readings differently for the purposes of liability, and a stilling officer whose volume drifts is answerable under the Ninth and blameless under the Fifth. The Circulation Division has instructed its officers to work to the Fifth and to cite the Ninth when questioned, a settlement that has held for eleven Cycles and is thought likely to collapse.

Records

Stilling certificates are filed by volume, not by floor, and the filing has never been reconciled with the numbering of the floors themselves. A certificate for a volume on Floor 47 may be found under any of four designations depending on its date, and requests for the earlier ones are routinely returned to the requester with the note that the volume in question was still. A register of volumes presumed still without certification is kept in the Cartographers' Spire and is not open to inspection.