Sentence:  When Frere had come down, an hour before, the prisoners were all snugly between their blankets. They were not so now; though, at the first <WSD>clink</WSD> of the bolts, they would be back again in their old positions, to all appearances sound asleep. 
Meaning of the word marked with <WSD> tags:  The sound of metal on metal, or glass on glass.