As he walked he had firmly tied one end of the string to the handle of the revolver. We had now reached the scene of the tragedy. With great care he marked out under the guidance of the policeman the exact spot where the body had been stretched. He then hunted among the heather and the ferns until he found a considerable stone. This he secured to the other end of his line of string, and he hung it over the parapet of the bridge so that it swung clear above the water. He then stood on the fatal spot, some distance from the edge of the bridge, with my revolver in his hand, the string being taut between the weapon and the heavy stone on the farther side.

“Now for it!” he cried.

At the words he raised the pistol to his head, and then let go his grip. In an instant it had been whisked away by the weight of the stone, had struck with a sharp crack against the parapet, and had vanished over the side into the water. It had hardly gone before Holmes was kneeling beside the stonework, and a joyous cry showed that he had found what he expected.

“Was there ever a more exact demonstration?” he cried. “See, Watson, your revolver has solved the problem!” As he spoke he pointed to a second chip of the exact size size and shape of the first which had appeared on the under edge of the stone balustrade.

“We’ll stay at the inn to-night,” he continued as he rose and faced the astonished sergeant. “You will, of course, get a grappling-hook and you will easily restore my friend’s revolver. You will also find beside it the revolver, string and weight with which this vindictive woman attempted to disguise her own crime and to fasten a charge of murder upon an innocent victim. You can let Mr. Gibson know that I will see him in the morning, when steps can be taken for Miss Dunbar’s vindication.”

Late that evening, as we sat together smoking our pipes in the village inn, Holmes gave me a brief review of what had passed.

“I fear, Watson,” said he, “that you will not improve any reputation which I may have acquired by adding the case of the Thor Bridge mystery to your annals. I have been sluggish in mind and wanting in that mixture of imagination and reality which is the basis of my art. I confess that the chip in the stonework was a sufficient clue to suggest the true solution, and that I blame myself for not having attained it sooner.

“It must be admitted that the workings of this unhappy woman’s mind were deep and subtle, so that it was no very simple matter to unravel her plot. I do not think that in our adventures we have ever come across a stranger example of what perverted love can bring about. Whether Miss Dunbar was her rival in a physical or in a merely mental sense seems to have been equally unforgivable in her eyes. No doubt she blamed this innocent lady for all those harsh dealings and unkind words with which her husband tried to repel her too demonstrative affection. Her first resolution was to end her own life. Her second was to do it in such a way as to involve her victim in a fate which was worse far than any sudden death could be.

"The house in the Square Lamartine is occupied by Prasville's men."

"Don't worry, Sebastiani. I shall get in. If they don't open the door, there's always the window. And, if the window won't open, I shall arrange with one of Prasville's men. It's a question of money, that's all. And, thank goodness, I shan't be short of that, henceforth! Good-night, Daubrecq."

He went out, accompanied by Sebastiani, and the heavy door closed after them.

Lupin at once effected his retreat, in accordance with a plan which he had worked out during this scene.

The plan was sunple enough: to scramble, by means of his rope, to the bottom of the cliff, take his friends with him, jump into the motor-car and attack d'Albufex and Sebastiani on the deserted road that leads to Aumale Station. There could be no doubt about the issue of the contest. With d'Albufex and Sebastiani prisoners; it would be an easy matter to make one of them speak. D'Albufex had shown him how to set about it; and Clarisse Mergy would be inflexible where it was a question of saving her son.

He took the rope with which he had provided himself and groped about to find a jagged piece of rock round which to pass it, so as to leave two equal lengths hanging, by which he could let himself down. But, when he found what he wanted, instead of acting swiftly - for the business was urgent - he stood motionless, thinking. His scheme failed to satisfy him at the last moment.

"It's absurd, what I'm proposing," he said to himself. "Absurd and illogical. How can I tell that d'Albufex and Sebastiani will not escape me? How can I even tell that, once they are in my power, they will speak? No, I shall stay. There are better things to try... much better things. It's not those two I must be at, but Daubrecq. He's done for; he has not a kick left in him. If he has told the marquis his secret, there is no reason why he shouldn't tell it to Clarisse and me, when we employ the same methods. That's settled! We'll kidnap the Daubrecq bird." And he continued, "Besides, what do I risk? If the scheme miscarries, Clarisse and I will rush off to Paris and, together with Prasville, organize a careful watch in the Square Lamartine to prevent d'Albufex from benefiting by Daubrecq's revelations. The great thing is for Prasville to be warned of the danger. He shall be."

The church-clock in a neighbouring village struck twelve. That gave Lupin six or seven hours to put his new plan into execution. He set to work forthwith.

When moving away from the embrasure which had the window at the bottom of it, he had come upon a clump of small shrubs in one of the hollows of the cliff. He cut away a dozen of these, with his knife, and whittled them all down to the same size. Then he cut off two equal lengths from his rope. These were the uprights of the ladder. He fastened the twelve little sticks between the uprights and thus contrived a rope-ladder about six yards long.

When he returned to this post, there was only one of the three sons beside Daubrecq's bed in the torture-chamber. He was smoking his pipe by the lamp. Daubrecq was asleep.