April gave place to May, and May to June, with no further development in the mystery which surrounded the Praed Street murders. The discovery of the knife in the pocket of Mr. Copperdock’s overcoat, combined with the fact that although Dr. Priestley and Mr. Ludgrove had each received a numbered counter, they had not so far been menaced by any particular danger, had convinced the police that the secret had died with Mr. Copperdock. A very careful examination of the handle showed that it bore his fingermarks, and his son, although he could not account for its presence where he had found it, was emphatic that it could not have been placed there since the night of his death.

The Professor made no reference whatever to the case, either to Harold or to Hanslet. The latter during his infrequent visits to the house in Westbourne Terrace, made no reference to the case. He was becoming more and more convinced that the Professor’s interview with the Black Sailor had been a pure hallucination; that the Professor, having imagined the sequel to his theory of the jurymen, had somehow been under the illusion that the events of his imagination had actually taken place. In his own mind, however, he was by no means satisfied with the official view of the matter, of which Whyland was the principal exponent. It left so much to be explained, chiefly Copperdock’s motive, to say nothing of his methods. To begin with, how had he contrived to murder Mr. Tovey, when at the moment of the latter’s death he was sitting comfortably before his own fire? And, if the answer to this was that he had accomplices, who and where were they?

However, his superiors appeared to be satisfied, and, towards the beginning of June Hanslet went off upon his long postponed leave. The Professor’s time was fully occupied in correcting the proofs of Some Aspects of Modern Thought, which were now coming in from the printer, whose reader, appalled by some of the Professor’s strictures upon his contemporaries, had scattered here and there across the margin the ominous words “Query, libellous?” To each of these the Professor added the note: “Nonsense!” in letters half an inch long at least. It was perhaps fortunate for the conscientious reader that he was beyond the reach of the Professor’s tongue.

The rigid police protection which had at first been imposed upon both the Professor and Mr. Ludgrove had been gradually relaxed until now it had dwindled to nothing more serious than a casual glance at their houses by the constable on beat when he happened to pass them. The professor himself appeared to have forgotten the warning given to him by the Black Sailor, and to have acquiesced in the official view that, since the perpetrator had committed suicide, nothing further would be heard of the murders.

It was while matters were in this state that Dr. Priestley received one Saturday morning a letter which obviously caused him some satisfaction. It was signed “Elmer Ludgrove” and was written in a curiously neat and clerkly hand. The contents of it were as follows:

Dear Sir. I should not venture to write to you, but for the fact that I know that you are as interested in the recent affair of the numbered counters as I am myself. I have recently come across, wholly by accident, certain curious facts which may serve to throw an entirely fresh light upon the deaths of six men, which, to my mind, have not yet been satisfactorily explained. I am not anxious to approach the police, at least until I have consulted you. The police, as is perhaps natural, could scarcely be expected to welcome disclosures which would completely upset their official theory. They believe, as I confess I did at one time myself, that the late Mr. Copperdock was guilty, and that his mania finally culminated in his own suicide.

If I am correct in my deductions from the new evidence which has come into my hands, I can prove beyond question that Mr. Copperdock was innocent, indeed, that he himself fell a victim to the agency which committed the other murders. I must ask you to respect my confidence until you have allowed me to put my evidence before you. When this has been done, I shall ask your advice as to the best means of laying it before the authorities. You, I have reason to believe, are on friendly terms with Inspector Hanslet, and perhaps you will agree, when you have examined the evidence I shall put before you, that it will be best to call him into consultation.

I suggest, as the various exhibits to which I shall be compelled to refer are at present at my house, and as it may be necessary for me to point out to you certain peculiarities of the locations in which the murders took place, that your examination be carried out there. As I believe that as little time as possible should be wasted, I suggest, if it meets with your approval, that you should do me the honour of calling upon me this evening. As, in the light of recent events, you may not care to come to Praed Street alone, I propose to call upon you after your dinner this evening, and to learn your wishes in the matter. If you agree to my suggestion, we can then walk to my house together. I am, sir, yours faithfully,

The Professor read this letter through twice. His final conclusion was that this man Ludgrove had in some way discovered that the dead men had all been members of the Morlandson jury. After all, he knew from the Black Sailor’s words that Ludgrove had made inconvenient enquiries, and it was possible that he had somehow stumbled upon the truth. Anyway, there could be no harm in going to see him and hearing what he had to say. Besides, he had never conveyed to him the Black Sailor’s warning. Hanslet might have, but the Professor, who had penetrated the ill-concealed scepticism of the Inspector, considered this unlikely. In any case, by comparing notes with this man, who had been living in Praed Street throughout the period of the murders, he might derive a hint which would lead him to his goal.

For Dr. Priestley, although he appeared to have put the whole affair out of his mind, was in fact determined to find the solution of the problem. He felt that so far the part he had played had been a distinctly ignominious one. Both his theory of the motive of the crimes and his account of his meeting with the Black Sailor had been disregarded. And although Dr. Priestley affected to regard his excursions into the solution of criminal problems with half-amused tolerance, his ill-success hitherto in this instance was a secret, but none the less bitter, blow to his pride.

Yes, he would certainly go to see this man Ludgrove. His letter showed him to be a man above the ordinary standard of education of people in his position. Hanslet had spoken of him as a man of considerable intelligence. The Professor, considering the matter, felt that he might well have got into touch with him before. He began to think that he had been too ready to rely upon the police descriptions of the events which had taken place, and had neglected an alternative source of information which might contain many details of value. And, his determination made, the Professor fidgeted all day until, having hurried through dinner and found himself in his study earlier than usual, he could expect his visitor.

Mary had been instructed to show him straight in, and, when he was at length announced, the Professor rose at once to greet him. Mr. Ludgrove, seen for the first time, was certainly an imposing and dignified figure. His snowy white hair and beard gave him a patriarchal appearance, produced an almost severe impression, which was only modified by the kindly smile which habitually hovered at the corner of his mouth. His eyes were screened by his powerful spectacles, but one guessed them to be direct and piercing in their expression.

“I am delighted to meet you, Mr. Ludgrove,” said the Professor warmly. “Yes, I am Dr. Priestley, and this is my secretary, Mr. Merefield. I received your letter, and am much obliged to you for your suggestion, which I shall be very glad to fall in with.”

“Thank you, Dr. Priestley, you are very kind,” replied Mr. Ludgrove, in his deep and pleasant voice. The Professor glanced at him in astonishment. Neither the voice nor the intonation were such as he would have expected of a herbalist in Praed Street.

“It is a beautiful evening,” continued Mr. Ludgrove. “Shall we walk to my house, or would you prefer to call a taxi? I know that the vicinity of Praed Street has a bad reputation as a result of these deaths, but although, as you probably know, I received a numbered counter some weeks ago, I have so far come to no harm. You need have no apprehensions once we reach my house. The police still display considerable concern for my safety.”

Mr. Ludgrove smiled, and even the Professor’s set features relaxed a little. “I am not afraid to walk to Praed Street, especially in your company, Mr. Ludgrove,” he replied.

“And coming back?” persisted the herbalist. “I fear I have no telephone, and it may take me some minutes to find you a cab. I should be happy to walk back here with you.”

The Professor made an impatient gesture. “Believe me, Mr. Ludgrove, I am no more afraid of Praed Street than you are. If you insist upon walking home with me, I shall be very glad of your company. If not, I am perfectly capable of taking care of myself.”

“It will be late, I am afraid, that is, if you have the patience to listen to me for so long,” replied Mr. Ludgrove. “I shall be compelled to inflict upon you a long and possibly tedious tale. However, you shall decide how much of it you wish to listen to.”

“I am usually considered a good listener,” said the Professor. “Shall we walk to your house, Mr. Ludgrove?”

The herbalist assenting, they left the house and walked down Westbourne Terrace together. Mr. Ludgrove’s shop was scarcely half a mile away, and they covered the distance in a few minutes, conversing about anything but the matter uppermost in their thoughts. The Professor found Mr. Ludgrove a most entertaining companion, and found himself wondering what such a man was doing to be compelled to keep a shop in such an odd neighbourhood.

They walked down Praed Street without adventure, and Mr. Ludgrove unlocked the door of his shop with his latchkey. They passed through it into the inner room, the window of which was already curtained and the gas lighted. Dr. Priestley cast a swift glance round it, noting with interest the scientific instruments on the bench, the cases full of books, and two comfortable armchairs. “This is a very comfortable room of yours, Mr. Ludgrove,” he remarked approvingly.

“It has heard some queer stories in its time,” replied Mr. Ludgrove with a smile. “You can form no idea of the extraordinary confessions which some of my clients force me to listen to. But I can assure you that it has heard nothing more amazing that the story which I am about to unfold to you this evening.”

“May I ask a question before you begin, Mr. Ludgrove?” said the Professor quietly.

“Certainly, pray ask as many as you please,” replied Mr. Ludgrove with a smile.

“How did you learn of my interest in this affair? So far as I am aware, it is not common property.”

“I am afraid that Inspector Hanslet was indiscreet,” replied Mr. Ludgrove. “When he first took over the case, he came here and had a long talk with me. He said then that he would put the facts before one whose powers of deduction were unrivalled. When I next saw him, he happened to mention your name, which I already knew as that of the boldest and most enterprising scientist of the age. It required very little intelligence on my part to infer that you would be interested in these extraordinary happenings, and that you were scarcely likely to accept the official explanation of them.”

“I see,” said the Professor. “Well, as it happens, your inference is correct. I am greatly interested, and I have a theory of my own as to the motive for the murders, which the police have been pleased to reject. I am all the more anxious to hear the evidence you have to put before me. There is still one more point in which I am interested. I am informed that, sometime before his death, Mr. Copperdock reported that he had met an individual known as the Black Sailor. Upon this being reported to you, you stated that you happened to have Mr. Copperdock under observation at the moment when he stated the meeting took place, and that no Black Sailor was present. Is this correct?”

“I can assure you, Dr. Priestley, that at that moment there was nobody in sight but Mr. Copperdock and myself,” replied Mr. Ludgrove simply.

“Thank you,” said the Professor. “Now, I am prepared to examine the evidence you have to show me.”

Mr. Ludgrove rose from his chair. “Will you excuse me for a few minutes, Dr. Priestley?” he said. “Before I begin my explanation, I should like to fetch certain things which I have put in a place of security upstairs. I will not keep you very long.”

He passed into the shop as he spoke, and the Professor could hear the sound of his footsteps as he climbed the stairs. The Professor, left alone in the room, proceeded to examine the titles of the books in the shelves. They were mainly botanical works, such as one might expect to find in the library of a herbalist. But here and there among them were a few volumes of general scientific interest. Dr. Priestley smiled as he saw, lying inconspicuously at the end of one of the shelves, a work he had published a year or so before, Fact and Fallacy. No doubt this had given Ludgrove the cue to his own interest in a problem so intricate as that of the Praed Street murders.

Lying next to it was a book bearing the name of an author well-known in the scientific world, which the Professor had never heard of. He took it up and began to turn over the pages with interest. His eye caught a passage dealing with one of the subtler mathematical problems raised by a particular aspect of relativity. He became absorbed in this, forgetting all about Mr. Ludgrove and his promised demonstration. So he remained for several minutes, until he had read the passage from beginning to end. Then, all at once, he became conscious that he was no longer alone in the room, that somebody was standing just inside the door, awaiting his pleasure. The Professor laid the book down, and turned on his heel, an apology upon his lips. But the words died in his throat, and he stood for an instant staring at the figure with unbelieving eyes.

It was the Black Sailor.