Morphen Cubit

The Morphen Cubit is a graduated rod of pale compressed bone, eleven handspans in length, divided into one hundred and forty-four morphen degrees and used to express the persistence rating of a subject on the Persistence Ladder. It is not a measure of length. The Cubit's markings record, at each degree, the resistance of a subject to being forgotten, and the rod is held against a thing — a doorway, a name, a stain on a stair — rather than laid beside it. Clerks of the Department of Anomalous Durations describe its reading as "the depth to which a subject resists the tap of an auditor" (Working Memorandum 1188, Circulating Copy). The instrument is distinct from the Morphen Cubit Mc, a later derivative whose additional suffix denotes a calibration class rather than a separate scale, and which has largely displaced the unmarked Cubit in field use without ever having formally replaced it.

Construction and graduation

Each Cubit is cut from a single stock of what the Vault of Primary Measures records only as "bone of the older sort," a material that is catalogued but not described. The rod is squared at the base and tapers to a groove on the upper face, which receives the index finger during reading. The 144 graduated degrees are unequal; the intervals widen in the lower range and narrow sharply beyond the 97th degree, so that the final thirteen degrees occupy the same span as the first four.

This is deliberate. Subjects rated above degree 97 have, in the Department's phrase, "begun to answer for themselves," and the graduations of the instrument are compressed there because the readings themselves are unreliable. Annex Ω of the Forty-Seventh Concordance fixes the proportion at 3:1 and forbids any recutting. A recut rod was found on Floor 802 in the winter of 1146; the attending clerk, Marda Opplin, had widened the upper graduations for legibility. The rod was withdrawn, Opplin was moved to inventory, and the episode is cited in the Annex as the reason the proportion "may not be improved."

The persistence rating

The Cubit yields a single figure, the persistence rating, expressed as a morphen degree with a subscript indicating the instrument class (unmarked, c for the certified field rods, p for the prototypical comparator). A rating is not an absolute. It is the departure of a subject from its own recorded echo, as measured against the Prototypical Cubit that is kept under glass in the Vault and brought out only for comparison readings, twice yearly, at the Central Scriptorium's Metrology Annex.

Ratings are used chiefly in disputes over whether a thing that has been removed from a floor remains present on it. The Bureau of Hypothetical Durability holds that a rating below degree 12 discharges the Tower of any obligation to the subject; the Division of Material Persistence reads the same figure as an instruction to keep searching. Both offices cite the Cubit, and neither has been persuaded by the other since the Motion on the Persistence of Unobserved Corridors was carried in the affirmative and then, three sittings later, in the negative.

Calibration and the Morphen question

The Cubit is calibrated against the Prototypical Cubit through a sequence of static echoes: the rod under test is brought into the presence of a subject already rated on the Prototypical and the resulting displacement is read back. The Third Concordance certifies the instrument in general terms and the Forty-Seventh Concordance gives the protocol; Concordant Law requires every certification to be recertified, and the recertification is what is actually relied upon in practice.

Where the two Concordances disagree is in what the Cubit is of. The Third states that the Cubit "measures persistence of occurrence, and no other thing" (Third Concordance, clause 41). The Forty-Seventh, written a century later, describes it as an instrument for the measurement of Ephemeral Gradients and of the unspooling and recoil adjacent to them, and adds that its readings "are to be understood as proximity, not as occurrence." The reconciliation entered in the Register of Readings holds that the Forty-Seventh amends the field and the Third amends the thing measured, which is either a distinction or a tautology, depending on the annotator. The School of Instrumental Realism has advanced a third view, that the Cubit measures neither but rather the disagreement between them, and has been asked to stop.

Drift and the Great Rereading of 1191

Because each rod is cut separately and then checked, the rods acquire small idiosyncrasies. The Department tolerates a drift of two-thirds of a morphen degree across the active stock and no more. The tolerance exists for the reason that tolerances usually do. During the Great Rereading of 1191 a batch of forty rods was issued from the Metrology Annex with the graduations cut from the wrong end — the taper on the base, the groove at the tip. The error went undetected for nine months because the readings produced were plausible, and plausible readings are not challenged.

When the batch was withdrawn, every subject measured by it had to be reread, and 611 subjects that had been certified as present were found to have been rated at their own echo and 4 that had been certified as absent were found to persist at degree 131. The 4 remain on the register. The 611 do not.

"The Cubit does not tell us whether a thing is there. It tells us how hard it is to stop asking."

— Marda Opplin, Notes from Inventory, Floor 802

Restrictions on use

The Cubit is a Primary Instrumental and may not be carried above Floor 1600 without an escort from the Vault. It may not be used to measure a person who is currently being measured by another Cubit, a rule introduced after a paired reading on Floor 44 produced two ratings that could not both be entered in the Register without erasing one of them. Rods found to be reading their own graduations rather than the subject are retired to the Vault's upper shelf and may not be repaired. Visiting clerks are asked to leave the rods in the rack when the lamps are trimmed.

Copies of the Cubit for instructional use are made from the Derived Cubit Rods and are marked with a groove at both ends so that no one mistakes them for the instrument. They are stored face-down.