The Hour of the Dragon by Robert E. Howard

CONAN LAY STILL, enduring the weight of his chains and the despair of his position with the stoicism of the wilds that had bred him. He did not move, because the jangle of his chains, when he shifted his body, sounded startlingly loud in the darkness and stillness, and it was his instinct, born of a thousand wilderness-bred ancestors, not to betray his position in his helplessness. This did not result from a logical reasoning process; he did not lie quiet because he reasoned that the darkness hid lurking dangers that might discover him in his helplessness. Xaltotun had assured him that he was not to be harmed, and Conan believed that it was in the man's interest to preserve him, at least for the time being. But the instincts of the wild were there, that had caused him in his childhood to lie hidden and silent while wild beasts prowled about his covert.

Even his keen eyes could not pierce the solid darkness. Yet after a while, after a period of time he had no way of estimating, a faint glow became apparent, a sort of slanting gray beam, by which Conan could see, vaguely, the bars of the door at his elbow, and even make out the skeleton of the other grille. This puzzled him, until at last he realized the explanation. He was far below ground, in the pits below the palace; yet for some reason a shaft had been constructed from somewhere above. Outside, the moon had risen to a point where its light slanted dimly down the shaft. He reflected that in this manner he could tell the passing of the days and nights. Perhaps the sun, too, would shine down that shaft, though on the other hand it might be closed by day. Perhaps it was a subtle method of torture, allowing a prisoner but a glimpse of daylight or moonlight.