Thus it was that with the shadows deepening about him, with his hopes fading one after another, Monsieur Mabeuf had remained serene, rather childishly but profoundly so. His spiritual states resembled the swing of a pendulum. Once set in motion by an illusion, the swing continued for a long time, even after the illusion had vanished. A clock does not stop the moment one loses the key.

[C’est ainsi qu’à travers cet obscurcissement qui se faisait autour de lui, toutes ses espérances s’éteignant l’une après l’autre, M. Mabeuf était resté serein, un peu puérilement, mais très profondément. Ses habitudes d’esprit avaient le va-et-vient d’un pendule. Une fois monté par une illusion, il allait très longtemps, même quand l’illusion avait disparu. Une horloge ne s’arrête pas court au moment précis où l’on en perd la clef.]

Victor Hugo (1802-1885) French writer

Les Misérables, Part 3 “Marius,” Book 5 “The Excellence of Misfortune,” ch. 4 (3.5.4) (1862) [tr. Denny (1976)]
    (Source)

(Source (French)). Alternate translations:

Thus it was that amid this darkness which was gathering about him, all his hopes going out one after another, Monsieur Mabeuf had remained serene, somewhat childishly, but very thoroughly. His habits of mind had the swing of a pendulum. Once wound up by an illusion he went a very long lime, even when the illusion had disappeared. A clock does not stop at the very moment you lose the key.
[tr. Wilbour (1862)]

It was thus that M. Mabeuf remained rather childishly but most profoundly serene, in the obscurity that was enveloping him gradually, and while his hopes were being extinguished in turn. The habits of his mind had the regular movement of a clock, and when he was once wound up by an illusion he went for a very long time, even when the illusion had disappeared. A clock does not stop at the precise moment when the key is lost.
[tr. Wraxall (1862)]

It is thus that, athwart the cloud which formed about him, when all his hopes were extinguished one after the other, M. Mabeuf remained rather puerilely, but profoundly serene. His habits of mind had the regular swing of a pendulum. Once mounted on an illusion, he went for a very long time, even after the illusion had disappeared. A clock does not stop short at the precise moment when the key is lost.
[tr. Hapgood (1887)]

So it was that amid this darkness gathering around him, all his hopes dimming one after another, M. Mabeuf had remained serene, somewhat childishly, but very deeply. His state of mind had the swing of a pendulum. Once wound up by an illusion, he went on a long time, even when the illusion had disappeared. A clock does not stop at the very moment you lose the key.
[tr. Wilbour/Fahnestock/MacAfee (1987)]

Thus, as the darkness gathered, as all his hopes died, one by one, Monsieur Mabeuf remained serenE, a little childishly, but profoundly so. His mind behaved like a swinging pendulum. Once wound up by an illusion, it kept going for a very long time, even after the illusion was gone. A clock does not stop dead the very moment the key is lost.
[tr. Donougher (2013)]