Great Ventilation Shaft

Great Ventilation Shaft is the collective designation for the four primary atmospheric trunks serving the Tower's middle floors, together with the maintenance apparatus, ceremonial observances, and litigation that have attached to them since the first of the four was opened in Cycle 41. The term is a category rather than a name: no single shaft is called the Great Ventilation Shaft, and the Ventilation Division has ruled twice (Bureau Note 912, Bureau Note 1144) that the phrase must not be used in the singular on signage. It is used in the singular on most signage.

The four trunks

The Great Shafts are numbered by the order in which their headings were read at the opening ceremony rather than by any physical property. They are the North Trunk, the Middle Trunk, the Ledger Trunk, and the Trunk of the Fourth Reading. The last is not a shaft in the ordinary sense but a sequence of eleven connected cells whose combined cross-section is treated as a single aperture for purposes of measurement. Circulation figures for the Tower proper are calculated against the total draught of all four, which is why the annual return has shown a headcount of "approximately" since Cycle 203.

Each trunk exchanges air with at least one subsidiary conduit. The North Trunk serves Ventilation Shaft 7 Gamma at its lower landing; the Middle Trunk draws its intake in part from the Aortic Shaft; the Ledger Trunk connects at gallery level with the Lambda Shaft; and the Trunk of the Fourth Reading is linked by a disputed coupling to Ventilation Shaft Theta, a coupling which the Seventh Concordance describes in Annex D as "intended" and which the Ventilation System registry describes as "not constructed."

Administration and the office of the Draught

Stewardship of the Great Shafts rests with the Ventilation Division, acting through the office of the Draught, a post held since Cycle 297 by the clerk Elianne Vosk, whose predecessor, Rennard Orm, resigned in Cycle 296 after an audit of the Ledger Trunk's pressure records disclosed that forty-one consecutive daily readings had been copied from a manual. Orm's letter of resignation is preserved in the Division's minute book and reads, in full: "I have no defence regarding the readings. I would note that the manual was accurate."

The office of the Draught maintains a staff of eleven ventilators and one recorder. The recorder is required to witness the reading of each trunk gauge in person and to initial the day's entry in ink. It is permissible to initial them all on the same day of the week. No entry may be made for a day not yet ended, with the exception of the Eve of the Fourth Reading, when the recorder must enter the following morning's figures before the ceremony begins, since the ceremony has on three occasions run past midnight.

The pressure strike of Cycle 218

The most significant interruption in the Great Shafts' history occurred in the winter of Cycle 218, when the ventilators of the North Trunk and the Middle Trunk withdrew their labour simultaneously over the Division's refusal to classify the draught at the ninth gallery as a hazard. The strike lasted nineteen days. During that period, air in the middle floors moved only through the Indirect Ventilation Conduit and the Great Northern Air Shafts, and the resulting condensation was heaviest on Floor 909, where the Department of Rested Instruments later recovered eleven pianos that had not been played since and could not afterwards be tuned to one another.

"The Division does not deny that the ninth gallery is cold. The Division denies only that cold constitutes a hazard. Cold is a condition of air; hazard is a condition of employment."

— Bureau Note 903, Ventilation Division

The strike was settled by the Eleventh Concordance, which created the post of gallery assessor and assigned the first assessor, Margit Halse, to the ninth gallery for a term of six cycles. Halse's report of Cycle 224 recommended no change and observed, in its closing line, that the gallery had "become no warmer, but has become considerably better known."

Ceremonial reading

Once each cycle, on the Eve of the Fourth Reading, the ventilators of all four trunks read aloud the opening clause of the trunk's founding instrument in the order their headings were first read. The ceremony is attested from Cycle 63 and has been suspended four times, twice for weather and twice because the instruments could not be located. The Ventilation Chorus attends and holds the pitch. Attendance by other divisions is by invitation of the Draught, and the invitations are numbered.

The recension question is unresolved. The Third Concordance directs that the reading follow "the earliest text of each instrument, without later gloss," while the Ninth Concordance directs that it follow "the text in current use." Since the two texts of the Middle Trunk differ at clause two — the earlier reading makes the shaft "a work of the Tower," the current one "a work for the Tower" — each Eve of the Fourth Reading produces a short but recorded dispute. In Cycle 288 the dispute was referred to the Librarians, who declined to resolve it and instead directed that both texts be read, one after the other, on alternate years. No year has yet been designated an alternate year.

Records and standing notes

The Great Shafts are the subject of some two hundred and forty pages of standing instructions in the Division's manual, most of which concern the closing of apertures during Ventilation Phases. The manual's most frequently cited clause is the fifteenth: "The North Trunk is not to be entered during Phase Two without the Draught's leave, however slight the reason." The most frequently violated clause is the same one.

Visitors to the middle floors are asked to note that the shafts are not a viewing gallery, and that the gratings at galleries nine, fourteen, and twenty are load-bearing only in the sense that they will bear one person at a time. Plans to widen the fourteenth gallery were proposed in Cycle 231, costed in Cycle 234, approved in Cycle 236, and are, as of this writing, filed.