"How did I manage to find the thing out?" said Cleek, answering the duke's query as they all sat together in the vicarage drawing-room, an hour or more afterward, discussing the affair. "Well, to tell you the truth, I think I got the very first inkling from the ringing of the bells. Oh, no, not through any special power of deduction or any super-human nonsense of that sort, but simply and solely through my memory. You will remember that I said Overton was not very original in his methods. Well, it so happened that I recalled the circumstance of a monkey which escaped from its cage and terrorized a Swiss village some years ago by climbing into the church belfry and inadvertently setting the bells ringing in the middle of the night by hopping from one to the other in an effort to maintain its equilibrium. That, of course, suggested a similar possibility here, and I set about deciding the point by means of the fullers' earth. By the way, Mr. Narkom, I chose fullers' earth not alone because of its yielding quality and its ability to receive impressions of even very slight things falling upon it, but because by reason of its weight and its rather tenacious character it was not so likely as another medium to be blown away by any sudden rising of the wind. I suppose you know that I placed the stuff about the tower's base when I left you at the limousine?"

"I judged that," replied the Superintendent. "The thing which amazed me, however, was all that meaningless stuff about timber and cubic feet you talked about when you came back. I can't even yet get the hang of that."

"Can't you? It will be getting in advance of the story a bit, but I may as well explain it now as later. I had come to the point of certainty regarding the making of a tunnel—how, you shall hear later—and as a tunnel of 288 feet in length will require shoring up and bracing with balks at intervals if one doesn't want it to become one's grave, timber for that purpose becomes a necessity. Now, as it would have been a pretty risky business to have left a trail by purchasing timber, and a worse one to attempt to smuggle it into the Hurdons' cottage, I had to figure out just where those beauties were going to get it. Old or new, they simply must have it, and the idea of old timber set my thoughts going—opened up, as it were, the possibility of the adjoining house being kept tenantless so that they could quietly demolish it and get the wood from there. So, when I went to lay the fullers' earth about the tower, I determined to have a look inside that other cottage and decide the point. A skeleton key let me in, and one flicker of my torch settled the question. The floors were up, the walls had been broken with a sledge-hammer until the place was fairly littered with mortar and lime from end to end, and every joist and beam that could be removed without bringing the place down had been taken away. I made a rapid inventory, calculated roughly about how much timber it would take to supply what had been removed, reduced that to the requirements of the tunnel, and when you caught me going about that little problem in mental arithmetic, I was trying to discover just how far those chaps must have progressed with the tunnel on the basis of the cubic feet of timber used.

"Now, to get back to the beginning. The tunnel idea was really the outcome of speculating with regard to Sir Julius Solinski's possible connection with the matter. That, of course, set me to thinking on the subject of digging for the purpose of getting something out of the earth by secret means and defeating the duke that way; then, when I got to thinking over Captain Paul Sandringham's position in the event of the proposed marriage and the possibility of an heir coming between him and his expectations, and added to that my knowledge of the dishonest character of the man—why, there you are! The tunnel and the reason for it simply evolve themselves by natural means out of such fertile soil as that; and, this much accomplished, it was but a step to the Hurdons and that very delightful garden which was so delightfully kept."

"How?"

"I didn't like the idea of that garden in that particular place for one thing, and then, for another, I didn't like its being so wonderfully kept up by a couple of people who couldn't apparently be scared into neglecting it or frightened out of the neighbourhood by all the ghosts and bells and curses in Christendom. I asked myself: 'Why do people of position and means so humble that they have to live in a little thatched cottage go to the expense of a garden so ornamental that even a ducal residence is enhanced by the presence of it?' The answer appeared to be that they were doing it for the purpose of making the duke pleased to have them remain there; and if they had any reason for spending a lot of money in plants and flowers simply to prevent the duke from tearing down the cottages and putting an end to the nuisance of that ill-kept one next door, there must be something at the bottom of it that would not stand looking into. If there was to be a tunnel, then, why should not that be the starting point? The more I thought over the thing the more promising it looked. For, I said to myself, to keep up a garden in the state of constant bloom and continual perfection, as that one appears to be, will need a lot of work; in fact, will keep that one old man and woman busy in it pretty much all their time, if there is no fake about it; but if there is, and they are digging in the tunnel, and the garden still bears those outward signs of constant care, why, there will be only one possible solution, one possible way to do it. In other words, there will have to be a constant supply of fresh plants that can be popped into the place of any that show the slightest trace of neglect, and that means that they will have to be shallow-rooted things like geraniums, fuchsias, lilies, and the like, so that the change cannot only be accomplished quickly, but will show no sign to the casual observer."

"Ah, now I understand what you meant by all that stuff about geraniums which I couldn't make head or tail of at the time. And that's why you were so interested and so anxious to go over and talk with the Hurdon woman the minute you caught sight of the place."

"That's it precisely, Mr. Narkom. I saw at once that I had hit upon something very like the truth; and as I also saw that the woman was very particular in rising from her work to see that her gardening apron was brushed down over her skirt, I wondered what she was so anxious to hide, and took means to find out by tossing the wall plant to her on the plea of its being a very choice plant. When she caught it in her apron and I saw those marks of caked yellow clay about her knees, I was both gratified and—disappointed."

"Disappointed, old chap?"

"Yes, in the colour of the stuff. I'd expected it to be blue. For, as you know, the county of Essex may be said to rest almost entirely on a foundation of what is geologically known as 'London clay' and that is decidedly blue. All in a moment, however, I remembered that we were close to the border line of Suffolk, and consequently on the edge of the yellow clay belt, and—there you are. Pardon, Duke? How did I get the idea of Captain Sandringham's connection with the affair? Oh, that fellow Carstairs gave me the first hint of that at the vicarage gate when the news of the man's death was made known, and Overton clinched the nail at once. He was obviously distressed by the news, and didn't know how to proceed; but when Carstairs flung out that little hint about there being 'one the less to reckon with' the fellow's eyes lit up in a manner not to be mistaken. They are expressive eyes. To-day, when I went over to the Castle and got the duke to send him off to Braintree so that I could slip into the Lodge and search his effects I found all the proof of 'Miss Emmy Costivan's' identity and of the beggar's arrangements with Captain Sandringham.

"I also made use of my time at the Castle to question the duke's valet and to learn what happened when Tom Davis went out to investigate the matter of the bells that night. Just before he went, Carstairs had treated him to half a bottle of the ducal champagne. When I heard that, I knew at once how the morphia had been administered. I had known from the first, however, that, if morphia should turn out to be the drug employed, Carstairs would be the man, because the rascal had it handy. How did I know that? Well, you remember that time when Sir Julius Solinski passed us in his motor? Carstairs saluted him. As the beggar put his hand to his hat, his sleeve slipped down, and I could see the evidence of the thing. The man was addicted to the use of hypodermic injections of morphia—the scars of the needle were clearly visible. I don't think there is any doubt of how poor Davis died. The drug overcame him on the road, and he dropped down in a stupor. There is clear evidence of that. You saw me examine his nails? The flint dust of the road was under them. After that, I suppose, Carstairs, who had been following him and watching him out for just such a time, pounced on him, carried him into the half-demolished cottage, and brained him with the hammer that was being used to batter down the walls and beat out the joists. As for the other tragic affair connected with the same cottage—namely, the mysterious disappearance of the child and the insanity and suicide of the father? I don't suppose we need regard that as other than a pleasant little fiction upon the part of Mr. James Overton. The duke tells me that the body of the man Smale was never recovered, and that his going insane and drowning himself rests only upon a 'farewell note' from him to Mr. Overton, who afterward declared that he had seen his body fished out of the river a good ten miles away. Of course the story of the child's vanishing in the dead of the night was another piece of fiction of the same sort. Nor would it surprise me in the slightest if the man called Smale, who posed as her father, was really no less a person than Captain Paul Sandringham himself.

"One thing, however, I think we can assert positively, and that is that Mrs. Mallory, the widow who set the ball rolling, was no other than Miss Emily Overton, otherwise Emily Costivan, and that the person who figured as her sister is the woman who passed as her mother."

"Mrs. Costivan?"

"Yes. I think we shall find that she and her husband were admitted into the game simply because they had at the time a nephew who was in the last stages of consumption. Of course the story about the gypsy who was beating him and who told the curse that would follow if he were buried here was made from whole cloth. The youth was simply carried into the house in the night, and Doctor Forsyth was right in the matter of his not having been able to lift a hand for days before he was called in to him. Since then it would appear that Mrs. Costivan had directed her energies toward washing the clay-stained overalls of one and the other of the 'tunnellers' when her husband carried them to her—for I dare say that every one of them—Carstairs, Overton, Costivan, all of them, took a hand in it at times; and if it hadn't been for poor Tom Davis, they might have carried it through successfully, after all. Which shows again that 'there's many a slip——!'"

And so ended the case of the Mysterious Light which nearly lost a duke a most valuable heritage and to which only Cleek could find the solution.