“What’s happened? Something very important. I ought to hope so after this confounded failure.”

“Failure? Didn’t he read the letters?”

“Yes, he read them. Had to, but—”

“Didn’t weaken? Eh?”

“No, he didn’t weaken. You can’t get water out of a millstone. You may squeeze and squeeze; but it’s your fingers which suffer, not it. He thinks we manufactured those letters ourselves on purpose to draw him.”

“Humph! I knew we had a reputation for finesse, but I didn’t know that it ran that high.”

“He denies everything. Said she would never have written such letters to him; even goes so far to declare that if she did write them—(he must be strangely ignorant of her handwriting) they were meant for some other man than himself. All rot, but—” A hitch of the shoulder conveyed Sweetwater’s disgust. His uniform good nature was strangely disturbed.

But Mr. Gryce’s was not. The faint smile with which he smoothed with an easy, circling movement, the already polished top of his ever present cane conveyed a secret complacency which called up a flash of discomfiture to his greatly irritated companion.

“He says that, does he? You found him on the whole tolerably straightforward, eh? A hard nut; but hard nuts are usually sound ones. Come, now! prejudice aside, what’s your honest opinion of the man you’ve had under your eye and ear for three solid weeks? Hasn’t there been the best of reasons for your failure? Speak up, my boy. Squarely, now.”

“I can’t. I hate the fellow. I hate any one who makes me look ridiculous. He—well, well, if you’ll have it, sir, I will say this much. If it weren’t for that blasted coincidence of the two deaths equally mysterious, equally under his eye, I’d stake my life on his honesty. But that coincidence stumps me and—and a sort of feeling I have here.”

It is to be hoped that the slap he gave his breast, at this point, carried off some of his superfluous emotion. “You can’t account for a feeling, Mr. Gryce. The man has no heart. He’s as hard as rocks.”

“A not uncommon lack where the head plays so big a part. We can’t hang him on any such argument as that. You’ve found no evidence against him?”

“N—no.” The hesitating admission was only a proof of Sweetwater’s obstinacy.

“Then listen to this. The test with the letters failed, because what he said about them was true. They were not meant for him. Miss Challoner had another lover.”

“Only another? I thought there were a half-dozen, at least.”

“Another whom she favoured. The letters found in her possession—not the ones she wrote herself, but those which were written to her over the signature O. B. were not all from the same hand. Experts have been busy with them for a week, and their reports are unanimous. The O. B. who wrote the threatening lines acknowledged to by Orlando Brotherson, was not the O. B. who penned all of those love letters. The similarity in the writing misled us at first, but once the doubt was raised by Mr. Challoner’s discovery of an allusion in one of them which pointed to another writer than Mr. Brotherson, and experts had no difficulty in reaching the decision I have mentioned.”

“Two O. B.s! Isn’t that incredible, Mr. Gryce?”

“Yes, it is incredible; but the incredible is not the impossible. The man you’ve been shadowing denies that these expressive effusions of Miss Challoner were meant for him. Let us see, then, if we can find the man they were meant for.”

“The second O. B.?”

“Yes.”

Sweetwater’s face instantly lit up.

“Do you mean that I—after my egregious failure—am not to be kept on the dunce’s seat? That you will give me this new job?”

“Yes. We don’t know of a better man. It isn’t your fault, you said it yourself, that water couldn’t be squeezed out of a millstone.”

“The Superintendent—how does he feel about it?”

“He was the first one to mention you.”

“And the Inspector?”

“Is glad to see us on a new tack.”

A pause, during which the eager light in the young detective’s eye clouded over. Presently he remarked:

“How will the finding of another O. B. alter Mr. Brotherson’s position? He still will be the one person on the spot, known to have cherished a grievance against the victim of this mysterious killing. To my mind, this discovery of a more favoured rival, brings in an element of motive which may rob our self-reliant friend of some of his complacency. We may further, rather than destroy, our case against Brotherson by locating a second O.B.”

Mr. Gryce’s eyes twinkled.

“That won’t make your task any more irksome,” he smiled. “The loop we thus throw out is as likely to catch Brotherson as his rival. It all depends upon the sort of man we find in this second O. B.; and whether, in some way unknown to us, he gave her cause for the sudden and overwhelming rush of despair which alone supports this general theory of suicide.”

“The prospect grows pleasing. Where am I to look for my man?”

“Your ticket is bought to Derby, Pennsylvania. If he is not employed in the great factories there, we do not know where to find him. We have no other clew.”

“I see. It’s a short journey I have before me.”

“It’ll bring the colour to your cheeks.”

“Oh, I’m not kicking.”

“You will start to-morrow.”

“Wish it were to-day.”

“And you will first inquire, not for O. B., that’s too indefinite; but for a young girl by the name of Doris Scott. She holds the clew; or rather she is the clew to this second O. B.”

“Another woman!”

“No, a child;—well, I won’t say child exactly; she must be sixteen.”

“Doris Scott.”

“She lives in Derby. Derby is a small place. You will have no trouble in finding this child. It was to her Miss Challoner’s last letter was addressed. The one—”

“I begin to see.”

“No, you don’t, Sweetwater. The affair is as blind as your hat; nobody sees. We’re just feeling along a thread. O. B.‘s letters—the real O. B., I mean, are the manliest effusions possible. He’s no more of a milksop than this Brotherson; and unlike your indomitable friend he seems to have some heart. I only wish he’d given us some facts; they would have been serviceable. But the letters reveal nothing except that he knew Doris. He writes in one of them: ‘Doris is learning to embroider. It’s like a fairy weaving a cobweb!’ Doris isn’t a very common name. She must be the same little girl to whom Miss Challoner wrote from time to time.”

“Was this letter signed O. B.?”

“Yes; they all are. The only difference between his letters and Brotherson’s is this: Brotherson’s retain the date and address; the second O. B.‘s do not.”

“How not? Torn off, do you mean?”

“Yes, or rather, neatly cut away; and as none of the envelopes were kept, the only means by which we can locate the writer is through this girl Doris.”

“If I remember rightly Miss Challoner’s letter to this child was free from all mystery.”

“Quite so. It is as open as the day. That is why it has been mentioned as showing the freedom of Miss Challoner’s mind five minutes before that fatal thrust.”

Sweetwater took up the sheet Mr. Gryce pushed towards him and re-read these lines:

“That to a child of sixteen!”

“Just so.”

“D-o-r-i-s spells something besides Doris.”

“Yet there is a Doris. Remember that O. B. says in one of his letters, ‘Doris is learning to embroider.’”

“Yes, I remember that.”

“So you must first find Doris.”

“Very good, sir.”

“And as Miss Challoner’s letter was directed to Derby, Pennsylvania, you will go to Derby.”

“Yes, sir.”

“Anything more?”

“I’ve been reading this letter again.”

“It’s worth it.”

“The last sentence expresses a hope.”

“That has been noted.”

Sweetwater’s eyes slowly rose till they rested on Mr. Gryce’s face: “I’ll cling to the thread you’ve given me. I’ll work myself through the labyrinth before us till I reach HIM.”

Mr. Gryce smiled; but there was more age, wisdom and sympathy for youthful enthusiasm in that smile than there was confidence or hope.