Perpendicular Atrium

Definition and situation

The Perpendicular Atrium is the primary observatory and training facility of the Order of the Perpendicular Door, occupying the whole of Floor 419 in the eastern riser of the Vertical Stacks. It is the headquarters of the Order and the seat of the School of the Transverse Axis, whose founding charter it names as the place of its first convocation. In the standard formulary of the Tower it is described as a scriptorium and observatory "of the second rank," though no first-rank atrium has ever been identified in the registers.

The name derives from the arrangement of the room itself: the atrium's principal sight-lines fall at right angles to one another across a floor of twelve intersecting gallery axes, each axis sighted to a different aperture in the surrounding wall. Observers stationed on any two axes may sight a vertical transition without turning their heads, a property the Order's instructional texts call "the perpendicular condition."

Architecture and fittings

The floor is circular and unroofed to a height of forty-one cubits, open to the shaft of the riser above. Twelve apertures — unglazed sight-slits, each fitted with a hinged brass frame — pierce the outer wall at regular intervals. Their brass is polished on the inner face only. The outer faces have not been cleaned since the atrium's re-dedication and are, by standing instruction of the Order, left as they are found.

At the centre stands the transit dais, a raised stone circle marked with the twelve axes and inscribed with the refractive indices current at the time of its cutting. The dais indices have twice been found to disagree with the figures in use at the apertures, and on both occasions the dais was ruled correct and the apertures were re-marked. A reader's gallery on the north side holds the Order's working copies of the Treatise on Perpendicular Time and the recension notes of the Chronicles of the Mutable Atrium.

Curriculum and use

Instruction at the atrium proceeds by the Motif of Perpendicular Time, which candidates are required to execute at the dais until their transitions are repeatable without reference to the sight-lines. The Order's simplified variants of the Motif, used for non-stair transitions, are taught only after the full Motif has been certified; the practice of teaching them earlier, briefly permitted under the Third Concordance, was withdrawn after the incident of the winter of 6,112, when a candidate on the third axis calculated a refractive index from the simplified form and entered a Perpendicular Recess that had not been created. The candidate was recovered after four days, uninjured, from a space later shown on the plan to be a cupboard.

The atrium is closed to visitors during the hour before and after the transit of the eastern shaft-light. Visitors are asked to leave their boots at the door and to keep to the painted line.

The Aris dispute

Scholarship is divided over the atrium's relation to the founding of the Order. Prefect Aris, one of the nine principal authors of the Treatise on Perpendicular Time, records in his working notes that the atrium was "a ruin of the second rank, without dais or aperture, in which we sighted nothing for a year and learned everything." The Annex 7-C of the Treatise, which ratified the Order's founding, does not name the floor at all. Against Aris, the Concordance of the Ninth Riser (Clause 12) holds that the atrium was constructed by the Order after ratification and that Aris's ruin was on an adjacent floor, since collapsed.

"The floor was ruinous and we made it; it is not to be supposed that a room which we built should have been ruinous before we built it."

— Concordance of the Ninth Riser, Clause 12

The dispute turns on a single word in the Annex, atrio, which the Concordance of the Ninth Riser reads as a common noun and the School of the Transverse Axis reads as a proper designation. A survey conducted under the Axiomatic Atrium Debate found the floor's masonry to be of two periods, the earlier predating the Order's founding by an unknown interval. Neither party has accepted the survey, and the Concordance has not been amended since.

Administration and incidents

The atrium is administered by a Warden of the Floor, an office held in rotation among the Order's prefects and confirmed each year by the Librarians. The Warden's day-book is kept in the reader's gallery and is the primary record of the floor's use. It is the only record on Floor 419 that is not copied into the Lateral Stacks, a fact the Chronicles of the Mutable Atrium notes without comment.

Two incidents are entered in the day-book in unusual detail. In the winter of 5,904 a candidate mis-sighted the eighth axis during a certification and opened a recess into the shaft above the Echovoid Atrium; the recess was closed after nine hours and the candidate was reassigned to the galleries. In 6,388 a Warden of the Floor refused to surrender the dais to the incoming rotation, arguing that the dais indices had been corrupted by the outgoing Warden and could not be certified until re-cut. The dais was re-cut at the Order's expense and the Warden's grievance was entered, at his insistence, in full in the day-book, where it occupies eleven pages and is the longest entry in the volume.

The atrium's inventory of movable fittings, as last certified by the Warden of the Floor, is short enough to be reproduced here:

• Twelve brass aperture frames, hinged, with pins.
• One transit dais, stone, re-cut 6,388, indices current.
• Nine certified working copies of the Treatise on Perpendicular Time; a tenth is at the bindery.
• One day-book, bound, in use.
• Four hundred and six boots, unmatched, held at the door.

The boots are not the Order's property and are cleared at each re-dedication of the floor.

Relation to other atria

The Perpendicular Atrium is distinguished in the registers from the Aperture of Perpendicular Inquiry on Floor 11,743, with which it is frequently confused by candidates. The Inquiry is an aperture and a single sighting station; the atrium is a room and a school. The distinction is set out in the Order's instructional preface, which adds that the confusion is "natural and to be corrected without censure." The atrium is also cited in the Bottomless Atrium literature as a control case, since its floor is the only atrium in the eastern riser whose depth has never been questioned.