Capillary Corridors

Capillary Corridors are transit passages of narrow bore through which the Tower's slower fluids are conducted between floors that share a pressure regime but not a datum. They are distinguished from the Volumetric Corridors by internal diameter — a Capillary Corridor is any passage whose clear width is at or below 1.4 metres — and from ordinary stairs by the fact that nothing is expected to walk through them. The distinction is administrative rather than physical, and the Concordance on Narrow Ways records that more than one clerk has been disciplined for treating it otherwise.

Capillary Corridors carry the hydration circuits that support Semi-Oneric Parchment, the dilute Silentium Resin suspensions used in gauge work, and the non-Newtonian inks drawn ultimately from the Capillary Well Sigma-44. Their terminus history is long and contested; the Delta-Sigma Capillary Cluster in particular intersects three Vertical Tram Lines and possesses no terminus rights in any of them, a condition confirmed in the Registry of Narrow Way Claims, Third Recension.

Classification and bore schedule

Bores are numbered by nominal internal width and grouped into four bands. Band A, to 0.4 metres, is reserved for gauge fluids and may not be entered by any instrument wider than the corridor's own test ring. Band B, to 0.8 metres, carries hydration. Band C, to 1.1 metres, carries ink. Band D, to 1.4 metres, is transitional, and the Annex of Perpetual Review has twice declined to rule on whether Band D corridors count as Capillary or Volumetric when empty — the question having arisen from a dispute in which the same corridor was assessed for tariff in both classes on the same afternoon.

Inspection is by the gauge, not the eye. The standard test admits a resin bead and times its passage; a corridor that returns its bead is struck from the schedule until recertified. A corridor that does not return its bead is also struck from the schedule, and a notice is posted at both ends.

Origin: the Second Wet Recension

The Corridors were not built as a network. They accumulated. The Second Wet Recension (Year 611 by the internal count, though the Annex of Perpetual Review dates it to 614 and the difference has never been settled) instructed that moisture reaching any floor must be returned to the nearest spire by the shortest possible route. "Shortest possible" was read, in the scriptoria of the eastern stacks, as "straightest line between the two nearest points of contact," an interpretation that generated corridors passing through the interior of several occupied floors. These remain in service. The Occupancy Accommodation Schedule of Year 627 regularized them by requiring that a bore crossing an occupied floor be sleeved, and that the sleeve be labelled at both ends with the bore number.

"Where a Capillary corridor crosses a floor to which a name has been assigned, the sleeve shall bear the borance number at each penetration, and no other inscription."

— Occupancy Accommodation Schedule, Clause 9

The prohibition on other inscriptions followed the affair of the Kestrel Sleeve, in which a clerk at the Delta-Sigma nexus engraved a small bird on each of forty-one sleeves as a private tally. The birds were found to have no hydraulic function and no disciplinary precedent, and were ordered effaced by the Librarian of the Lower Recesses over the objection of the clerk, who had numbered them consistently and could account for all forty-one.

The Theta Aperture dispute

The Aperture Theta is a constriction on a Band C corridor not far below the Delta-Sigma Capillary Cluster, and is named in the Inventory of Constricted Sections as the narrowest certified passage in the western stack: 0.27 metres at the throat. Two readings govern it. The Concordance on Narrow Ways treats an aperture as a property of the corridor, so that a blockage at Theta is a blockage of the whole; the Third Concordance, Annex 4, treats it as a separate holding with its own inspector and its own tariff. Under the first reading the corridor is one asset subject to one assessment. Under the second it is two assets joined by a third.

The practical consequence arose in the winter of the Year of the Long Frost, when the Theta throat iced and the upper stretch failed to drain. The upper inspector certified the failure as originating below Theta, and therefore not his. The lower inspector certified it as originating above Theta, and therefore not hers. The corridor remained frozen for eleven days, during which a quantity of Band C ink settled out and was written off. A resolution in the Accounts of Struck Assets, Volume 14 assigned the loss to both offices in equal parts, and neither has accepted it.

Relation to Mirror and Volumetric works

Capillary Corridors are occasionally absorbed into larger projects. The Creation of Mirror Corridors protocol permits a Band D corridor to be reclassified as a Mirror-Stacks spur where the Motion on the Persistence of Unobserved Corridors has not been invoked against it. Similarly, a Capillary Corridor whose throughput exceeds its bore class may be nominated for conversion under the Volumetric Corridors procedure; conversion requires a surveyor, a witness, and the surrender of the corridor's original borance number, the last of which is the customary reason conversions stall. Where conversion does proceed, the corridor is added to the records held on Floor 1004 alongside the memorials placed under Memorialization Protocol Alpha in lesser-used passages branching from Corridors Alpha through Epsilon.

A corridor may be memorialized without being converted. The practice is discouraged but lawful, and the inscriptions permitted under the Protocol are the only inscriptions other than sleeve numbers that the Occupancy Accommodation Schedule tolerates.

Maintenance and current standing

The Capillary Corridors are maintained by the Office of Narrow Conduits and inspected on a six-year cycle, offset from the tram survey so that no two inspections fall in the same quarter. Rodents are not the difficulty. Capillary Corridors are too narrow for most of them and too wet for the rest. The difficulty is scale, which forms at every straight run of more than nine metres and must be scraped before the bore is lost; the scraping is done by a weighted brush on a wire, and the brush may not be withdrawn through an occupied floor. The Atlas of Oneiric Water Tables is consulted before any scraping near a corridor of Band B or above, on the grounds that the water tables beneath the corridors and the tables catalogued in the Atlas have on three recorded occasions been found to correspond, and the correspondence has never been explained.

As of the Annual Return of Certifications, 1,412 Capillary Corridors stand certified. Of these, 388 are Band D and their classification is regarded at the Annex as provisional. The Tower's position, stated in the Concordance on Narrow Ways and repeated without alteration for eleven recensions, is that the narrow ways are older than the question of what to call them, and that they will outlast it.