I tried my best, but there was very little that I could find out about Mrs. Barry. No one seemed to know where she came from, and even "Mr. Barry" seemed shrouded in obscurity. I was convinced, however, that she was an adventuress.

One thing, however, I did turn up. She had called on Tresham at his office a number of times, usually late in the afternoon, and he had taken her to dinner and to the theater. Apparently he knew her a great deal better than he had been willing to admit to us. I was not surprised, for, like a good many men of his class, Tresham was better known in the white light district than one might suspect. Mrs. Barry had all the marks of being good company on such an excursion.

On the way uptown, I stopped off in the neighborhood of Longacre Square in the hope of picking up some more gossip at one or another of the clubs. Tresham was a member of several, though as near as I could find out, used them more for business than social reasons. On Broadway it was different, however. There he was known as a liberal spender and lover of night life. Like many others he now and then accumulated quite large bills. I wondered whether Mrs. Barry had not found out and taken advantage of his weakness.

It was, as I have said, comparatively little that I had been able to discover, yet when I met Kennedy again, later in the evening, at his laboratory, he listened eagerly to what I had to report.

"Did anything happen downtown?" I asked when I had finished.

"Nothing much," he returned. "Of course, listening over the geophone, I couldn't watch the Bank Building, too. There's something very queer about Creighton. I could hear him at work in the room upstairs until quite late, making a lot of noise. If I don't find out anything more definite soon, I shall have to adopt some other measures."

"You didn't do anything more about that electrolysis clew?" I queried.

"Nothing," he replied briefly, "except that I inquired of the electric light company and found out that Creighton, or someone in his building, was using a good deal of power."

"That looks bad," I ventured, remembering the claims made for the engine and the comparatively weak batteries that were said to run it.

Kennedy nodded acquiescence, but said nothing more. We walked over in silence to our apartment on the Heights and far into the night Craig sat there, shading his eyes with his hand, apparently studying out the peculiar features of the case and planning some new angle of approach at it tomorrow.

We were surprised the next day to receive an early visit from Miss Laidlaw at the laboratory. She drove up before the Chemistry Building, very much excited, as though her news would not bear repeating even over the telephone.

"What do you think?" she exclaimed, bursting in on us. "Mr. Creighton has disappeared!"

"Disappeared?" repeated Kennedy. "How did you find it out?"

"Mr. Tresham just telephoned me from his office," she hurried on. "He was going into the Bank Building when he saw a wagon drive off from the place next door. He thought it was strange and instead of going on up to his own office he walked into Creighton's. When he tried to get in, the place was locked. There's a sign on it, too, 'For Rent,' he says."

"That's strange," considered Kennedy. "I suppose he didn't notice what kind of wagon it was?"

"Yes, he said it looked like a junk wagon—full of stuff."

I looked from Miss Laidlaw to Kennedy. Plainly our entrance into the case had been the signal for the flitting of Creighton.

Quickly he reached for the telephone. "You know Mrs. Barry's number?" he asked.

"Yes, it's the Prince Edward Hotel."

He called up, but the conversation was over in a moment. "She didn't return to the hotel last night," he announced as he hung up the receiver.

"She's in this thing, too," exclaimed Adele Laidlaw. "Can you go down with me now and meet Mr. Tresham? I promised I would."

Though she repressed her feelings, as usual, I could see that Adele Laidlaw was furious. Was it because Creighton had gone off with her money, or was it pique because Mrs. Barry had, perhaps, won him? At any rate, someone was going to feel the fury of her scorn.

We motored down quickly in Miss Laidlaw's car and met Tresham, who was standing in front of the Bank Building waiting for us.

"It just happened that I came down early this morning," he explained, "or I shouldn't have noticed anything out of the way. The junk wagon was just driving away as I came up. It seemed to be in such a hurry that it attracted my attention."

It was the first time we had seen Tresham and Miss Laidlaw together and I was interested to see how they would act. There was no mistaking his attitude toward her and Adele was much more cordial to him than I had expected.

"While I was waiting I got a key from the agent," he explained. "But I didn't want to go in until you came."

Tresham opened the door and led the way upstairs, Miss Laidlaw following closely. As we entered Creighton's shop, everything seemed to be in the greatest disorder. Prints and books were scattered about, the tools were lying about wherever they happened to have been left, all the models were smashed or missing and a heap of papers in the fireplace showed where many plans, letters and other documents had been burned.

We hurried into the big room. Sure enough, the demon motor itself was gone! Creighton had unbolted it from the floor and some holes in the boards had been plugged up. The room below was still locked and the windows were covered with opaque paper on the inside.

"What do you suppose he has done with the motor?" asked Adele.

"The only clew is a junk dealer whom we don't know," I replied, as Kennedy said nothing.

We looked about the place thoroughly, but could find nothing else. Creighton seemed to have made a clean getaway in the early hours.

"I wish I could stay and help you," remarked Tresham at length. "But I must be in court at ten. If there's anything I can do, though, call on me."

"I'm going to find that engine if I have to visit every junk dealer in New York," declared Miss Laidlaw soon after Tresham left.

"That's about all we can do, yet, I guess," remarked Kennedy, evidently not much worried about the disappearance of the inventor.

Together we three closed up the workshop and started out with a list from a trade publication giving all those who dealt in scrap iron and old metal. In fact we spent most of the day going from one to another of the junk shops. I never knew that there were so many dealers in waste. They seemed to be all over the city and in nearly every section. It was a tremendous job, but we mapped it out so that we worked our way from one section to another.

We had got as far as the Harlem River when we entered one place and looked about while we waited for someone in charge to appear.

I heard a low exclamation from Kennedy, and turned to look in the direction he indicated. There, in a wagon from which the horse had been unhitched, was the heavy base of the engine into which so many dollars had been turned—sold as so much scrap!

Kennedy examined it quickly, while I questioned a man who appeared from behind a shed in the rear. It was useless. He could give no clew that we already could not guess. He had just bought it from a man who seemed anxious to get rid of it. His description of the man tallied with Creighton. But that was all. It gave us no chance to trace him.

"Look," exclaimed Kennedy eagerly, bending closer over the motor. "This is one of the neatest perpetual motion frauds I ever heard of."

He had turned the heavy base of the motor upward. One glance left me with little wonder why Creighton had so carefully bolted the machine to the floor. In the base were two rectangular apertures to allow a belt to run over a concealed pulley on the main shaft of the machine in the case. Evidently, when the circuit from the Daniell cells was closed, the pulley, somehow, was thrown into gear. It was loose and the machine began to revolve slowly at first, then faster and with great show of power. The pounding, as Kennedy had surmised, was due to the flywheel not well balanced.

"Well," I remarked, "now that we have found it, I don't see that it does us much good."

"Only that we understand it," returned Craig. "I left that geophone down there in the room next door which I hired. I think, if Miss Laidlaw will take us down there, I'd like to get it."

He spoke with a sort of easy confidence which I knew was hard to be assumed in the face of what looked like defeat. Had Craig deliberately let Creighton have a chance to get away, in order that he might convict himself?

In silence, with Miss Laidlaw at the wheel, we went downtown again to the room which Craig had hired next to Creighton's workshop. As we approached it, he leaned over to Miss Laidlaw.

"Stop around the corner," he asked. "Let's go in quietly."

We entered our bare little room and Kennedy set to work as though to detach the geophone, while I explained it to our client.

"What's the matter?" she interrupted in the middle of my explanation, indicating Kennedy.

He had paused and had placed the receivers to his ears. By his expression I knew that the instrument was registering something.

"Someone is in the lower room of the shop next door," he answered, facing us quickly. "If we hurry, we'll have him cornered."

Miss Laidlaw and I went out and around in front, while Craig dashed through a back door to cut off retreat that way.

"What's that? Hurry!" exclaimed Miss Laidlaw.

Plainly there was a muffled scream of a woman as we entered the street door. I hurried forward. It was the work of only a few seconds to batter down the locked door in the room under Creighton's old workshop, and as the door gave way, I heard the sound of shattered glass from the rear which told that Kennedy had heard the scream, too, and had gained an entrance.

Inside I could make out in the half-light a man and a woman. The woman was running toward me, as if for help.

"Mrs. Barry!" gasped Adele Laidlaw.

"He got me here—to kill me!" she cried hysterically. "I am the only one who knows the truth—it was the last day—tonight he would have had the money—and I would have been out of the way. But I'll expose him—I'll ruin him. See—he came in from the roof—"

A blinding flash of light greeted us, followed by a scream from Adele Laidlaw, as she ran past us and dropped on her knees beside a body that had fallen with a thud in the flame before a yawning hole in the side wall.

Mrs. Barry ran past me, back again, at almost the same moment. It was a strange sight—these two women glaring at each other over the prostrate figure of the man.

"Here's the real demon engine," panted Craig, coming up from the back and pointing to an electric motor as well as other apparatus consisting of several series of coils. "The perpetual motion machine was just a fake. It was merely a cover to an attempt to break into the bank vaults by electrolysis of the steel and concrete. Creighton was a dummy, a fiction—to take the blame and disappear when the robbery was discovered."

"Creighton," I repeated, looking at the man on the floor, "a dummy?"

"Oh—he's dead!" wailed Adele Laidlaw. "He's dead!"

"Electrocuted by his own machine rather than face disgrace and disbarment," cut in Craig. "No wonder she was in doubt which of the two men fascinated her most."

I moved forward and bent over the contorted form of the lawyer, Tresham, who was wearing the whiskers and iron gray wig of his alter-ego, Creighton.