At last, when the ship drew near to the outskirts, as it were, of the Equatorial fishing-ground, and in the deep darkness that goes before the dawn, was sailing by a cluster of rocky islets; the watch—then headed by Flask—was startled by a cry so plaintively wild and unearthly—like half-articulated wailings of the ghosts of all Herod’s murdered Innocents—that one and all, they started from their reveries, and for the space of some moments stood, or sat, or leaned all transfixedly listening, like the carved Roman slave, while that wild cry remained within hearing.
Melville's use here of the definite article (the not a) indicates one specific work, evidently a marble statue of some kind, or bust, although the object is not further particularized. I take this work of art to be real and pretty well-known, famous enough that Melville could expect readers to recognize it with minimal descriptive fussing on his part. The biggest clue to the thing he means is conveyed in the main point of comparison between sailors and the marble slave, their shared attitude as rapt listeners. As Ishmael relates, upon hearing an eerie cry in the dark, "plaintively wild and unearthly," the whale-men on watch froze like statues. For a time they remained stationary, "transfixedly listening, like the carved Roman slave." Via Ishmael's simile, Melville thus compares the crew's motionless bewitchment to that exhibited by an apparently familiar work of visual art. Although unnamed, whichever "carved Roman slave" Melville had in mind is introduced in the Life-Buoy chapter of Moby-Dick as a model of eternally attentive listening.
In his end-notes for the 1972 Penguin edition of Moby-Dick, Harold Beaver suggested that Melville's "carved Roman slave" might designate "the 'Dying Gaul' often called the 'Dying Gladiator.'" Online, the Melville Electronic Library offers a textual note that makes similarly tentative connections to the famous Dying Gaul or Dying Gladiator in the Capitoline Museum. In earlier writings (Mardi and White-Jacket) Melville had referred specifically to the Dying Gladiator, so clearly he knew of it. But the sculpture alluded to in Moby-Dick has a characteristic, fixed expression of listening (Melville's word) that the suffering figure of the Gaul manifestly lacks. Mortally wounded, the fallen warrior looks downward, however intently. In his sad condition the Dying Gaul or Gladiator is always dying, rather than listening.
The far more likely candidate for Melville's "carved Roman slave" is the well-known sculpture at the Uffizi Galleries in Florence called the Arrotino or Knife-Grinder, also known as "The Scythian." Both the statue and a place Melville might have seen it in some form before writing Moby-Dick have been positively identified by Hershel Parker in a footnote to Chapter 126 for the 3rd Norton Critical Edition of Moby-Dick, on page 376:
"In London, Melville could have seen a cast of this famous statue, which is in the Tribune of the Uffizi Museum, in Florence, Italy."
Melville had visited London and the Continent in 1849-1850.
The sculpture is well described by Cristiana Barandoni on the Uffizi Galleries website:
The sculpture, sold by the Mignanelli family to Cardinal Ferdinando de’ Medici, and was bought to Florence in 1677 and placed on display in the Tribune, where it can still be seen. It is known as the “Arrotino” and shows a kneeling man who is sharpening a knife on a stone. The man, who has long eyebrows, recessed pupils and swollen eyelids, is looking upwards, his forehead marked with deep frown lines. The semi-naked figure, wearing a light cloak over his right shoulder, was initially thought to be a Scythian, or even a royal barber plotting against the state. In the 16th century, the idea was put forward that the sculpture could be part of a group depicting the flaying of Marsyas. The figure was therefore identified as a slave, preparing the blade used to torture the satyr.
Joseph Addison, in frequently reprinted Remarks on Several Parts of Italy, influentially described the Arrotino as "the Roman Slave whetting his Knife and listening." Then as now, the sculpture was exhibited in the Tribune, Uffizi Gallery with other world-famous works including the celebrated Medici Venus, Dancing Faun or Dancing Satyr, and Wrestlers:
In the same Chamber is the Roman Slave whetting his Knife and listening, which from the Shoulders upward is incomparable. The two Wrestlers are in the same Room.
Guidebooks of Melville's time noted that the Arrotino was
"called also the Knife-grinder and the Listening Slave"
-- Italy by Josiah Conder Volume 3 (London, 1834) page 373.
Melville and his contemporary readers took the sculpted Arrotino for a "Roman slave," as Addison had called him. In England and also America the carved figure was called "The Listening Slave." Considered together with "Roman slave," this formerly popular but now forgotten name for the Knife-Grinder perfectly explains Melville's choice of words in the Life-Buoy chapter, when Ishmael depicts startled sailors as "transfixedly listening" and then compares them in that regard to the statue of a Roman slave known as
THE LISTENING SLAVE.
Shown below, the figure of "The Listening Slave" as illustrated on the front page of the Saturday Magazine Supplement in February 1842:
Architect James Hakewill noted the popularity of the "Listening Slave" title in England:
... the Arrotino, or the Grinder, commonly known in England under the name of the Listening Slave, has been supposed by some to have been raised in honour of a slave who detected the secret machinations of the Catilinarian conspiracy. Nothing however is really known relative to the original design of the artist, but its taste and execution are such as seem worthy of the best sculptors of Greece. -- A Picturesque Tour of Italy
In New York City, the National Academy of Design owned a reproduction they called "The Listening Slave," listed as #166 in the 1846 Catalogue of Statues, Busts, Studies, Etc. Forming the Collection of the Antique School.
If he never saw a copy of "The Listening Slave" in Manhattan, or London, Melville eventually got to view the original in Florence. At the Uffizi on March 26, 1857 Melville was "not pleased with the Venus de Medici" but "very much astonished at the Wrestlers" he would have seen in the Tribune gallery. He must have examined the Arrotino there, too, one of numerous other works of art he regarded as "Idle to enumerate" after repeated tours of the Uffizi Galleries. Documented in Melville's Journal of a Visit to Europe and the Levant, edited by Howard C. Horsford (Princeton University Press, 1955) page 218; also the 1989 Northwestern-Newberry edition of Melville's Journals, edited by Horsford with Lynn Horth, page 115.
"As to picturesqueness of character, or that quality which is best suited to a picture, there has been so much already written, and though not very conclusively, yet so well, on the subject, that it is a fearful matter to touch upon it again. One point, however, may be adverted to, as being that, which will be found in a great degree conducive to this end. If we investigate with attention the works of the ancient sculptors, we shall discover a peculiarity in their practice, which has not been generally noticed, and this regards the time of action selected by them as fittest for their purpose. It is never the middle of an action that is represented, but in every example a momentary pause, or suspension of motion: and this, it will be seen, may be so chosen, as to give the fullest perception of all that has immediately preceded, or, in other words, to tell the story. Thus the Apollo Belvedere is not exhibited as if in the act of shooting; but the arrow is already gone, and he rests for a moment, following its flight with his eye: even the figure of the Laocoon is not represented actually in motion, but the moment given is the end of one of the paroxysms of his agony, when he is for a while fixed: the same may be observed in the fighting gladiator, in the listening slave, and all the greatest works of
antiquity. This principle may be applied most strictly also to painting, and we shall observe the same momentary pause of action to have been purposely selected by all the great masters of design. A figure of Raffael, or M. Angelo, &c. is never drawn as if actually moving; but the point taken is during a momentary stagnation of action, or while they are for an instant rapt, if the phrase may be allowed."
In the manner of Raffael, Michelangelo, or the unknown sculptor who made the carved Roman slave variously called the Arrotino, Knife-Grinder, or Listening Slave, Melville drew his whalemen during a pause when, hearing the crying of young seals nearby, probably, they were (to apply the formulation of ideal picturesqueness offered in 1820 by J. T. James) "for an instant rapt."