Prospero Marble

Prospero Marble is a veined carbonate stone of the Tower's upper annexes, distinguished from the other acoustically active marbles not by what it emits but by what it declines to emit. Where Lament Marble sustains a lament-frequency tone and Ignorance Marble produces the higher, paler scream that characterizes the Screaming Pillars, Prospero Marble absorbs incident sound almost entirely and returns it after a delay so long that most auditors have left the gallery before it arrives. The stone is quarried in small lots and has never been catalogued in quantity.

Acoustic character

Prospero Marble is grey-green with a close, wavy veining of pale quartz and a fine dispersal of pyrite that gives polished surfaces a dull gold cast under lamplight. Its absorption coefficient, measured at the Standard Bracket test gallery on Floor 208, stands at 0.91 across the middle register, the highest recorded for any Tower marble. The returned residue is not silence but a lag: struck surfaces give back their note at intervals of eleven to forty minutes, in fragments, at reduced amplitude.

Workers in the upper annexes call this the afterword. The Office of Audible Properties has not adopted the term, though it appears without comment in three survey returns from Floor 209.

"A sample struck at the fourth bell returned its tone during the fifth, and again, faintly, at the change of the watch. No further emission was recorded. The stone is not silent. It is slow."

— Survey Return 209/14, Office of Audible Properties

The stone is frequently confused with Whispering Marble, which is quarried at Floor -17, Annex Theta and whose Echo-Shelf phenomenon depends on a rapid, continuous return. Prospero Marble's return is neither rapid nor continuous, and the Third Concordance's Survey Appendix B does not list it among the veining classes it distinguishes, an omission discussed below.

Quarrying and supply

The known workings are the Prospero Face, a shallow gallery on Floor 211 opened in the winter of the year the Tower counts as 412. The face yields blocks of irregular size and considerable weight; a block measuring one ell by two is recorded at forty-one stone, which is heavier than the hauling crews on 211 will move without a second Bracket inspection. Consequently most Prospero Marble in circulation is small: plinths, sill plates, the occasional door surround in the Wandering Gallery.

Supply has never been regular. The face is worked only when the upper annexes require a soundproofed chamber, and the Office of Audible Properties has twice ordered the gallery closed to allow the stone to "settle," a term the Office has declined to define. The closures are recorded in the quarry ledgers but not in the general registers, which is the origin of the present dispute over holdings.

The naming dispute

Prospero Marble takes its name from Clerk Prospero, who surveyed the upper annexes for the Office of Audible Properties in the years following the face's opening. Clerk Prospero's survey notebooks describe the stone at length and recommend its use in the Vaults of the Fifth Register, where the reading rooms adjoin a stairwell of Lament Marble and the two tones were said to interfere. The recommendation was adopted. Clerk Prospero did not receive credit for the name, which was assigned by the Nomenclature Subcommittee on the ground that the stone "promises sound and withholds it," a characterization Clerk Prospero disputed in a memorandum of some length and no consequence.

The grievance is preserved in the Subcommittee's own file. Clerk Prospero's memorandum argues that the name should properly attach to the stone's absorbent quality rather than its delayed return, and that the Subcommittee had, in the clerk's words, "named the stone for its manners and not its matter." The memorandum was filed without reply. Clerk Prospero's subsequent surveys of the face are unsigned.

Competing recensions

Two positions govern the stone's standing in the archive, and they do not agree.

The first holds that Prospero Marble is a variety of Whispering Marble, its lag being merely an extreme expression of the Echo-Shelf, and cites the shared quartz veining and the proximity of the upper annexes to the Theta workings as evidence. This position is set out in the Fourth Concordance, Annex 2, which lists Prospero Marble as "Whispering, retarded."

The second holds that the stone is a distinct class and that its absorption profile excludes it from the Whispering family altogether. Its advocates cite the Third Concordance's silence on the subject not as an omission but as a deliberate exclusion, and point to the absence of any recorded Echo-Shelf measurement from the Prospero Face. The Office of Audible Properties has endorsed neither position in writing. Asked to comment for the present entry, the Office returned a slip reading only "Noted."

A third view, advanced by the Keeper of the Upper Annexes and supported by no document, holds that the stone is Sorrow-Stained Marble weathered to a different colour. The Keeper has not repeated the claim since the condensate question was raised at the Assizes of the Ninth Stair.

Use in the Tower

Prospero Marble is valued chiefly for its absorption. The Vaults of the Fifth Register are lined with it to a height of two ells, above which the original Lament Marble of the stairwell remains exposed; readers in the adjoining rooms report that the stairwell's tone arrives "from a great distance," though the stairwell is nine feet away. The Silence of the Third Landing is attributed to the same lining, though the landing itself is floored in a different stone and the attribution is contested.

Small pieces are used as Standard Tone Absorbers in the Bracket test galleries, where they are struck before each measurement and then set aside for the duration of the session. A maintenance instruction of the Office of Audible Properties directs that these absorbers be stored face down and not stacked, as the afterword of one piece will otherwise be mistaken for the emission of the piece above it. The instruction is observed.

The stone does not take a polish well and is seldom used where it will be seen. Its principal decorative employment is the Prospero Door of the upper annexe scriptorium, which is painted, and the sill plates of the Wandering Gallery, which are not.

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See also Controversy of the Empty Niche, which concerns a Prospero Marble plinth in the upper annexes whose displayed object has never been identified, and Incident on Floor 331, where a quantity of the stone was found in a corridor that the quarry ledgers record as never having supplied it.