hutchinson hatch gathered up his overcoat and took the steps coming down two at a time. there was no car in sight, nothing on wheels in fact, until — yes, here was an automobile turning the corner, an automobile cab drifting along apparently without purpose. hatch hailed it.

“get me out to commonwealth avenue and arden street in a hurry!” he instructed. “take a chance with the speed law, and i’ll make it worth while. it’s important.”

he yanked open the door, stepped in, and closed it with a slam. the chauffeur gave a twist to his lever, turned the car almost within its length, and went scuttling off up street.

safely inside, hatch became suddenly aware that he had a fellow passenger. through the gloom he felt, rather than saw, two inquisitive eyes staring out at him, and there was the faintest odor of violets.

“hello!” hatch demanded. “am i in your way?”

“not in the slightest,” came the voice of a woman. “am i in yours?”

“why — i beg your pardon,” hatch stammered. “i thought i had the cab alone — didn’t know there was a passenger. perhaps i’d better get out?”

“no, no!” protested the woman quickly. “don’t think of it.”

then from outside came the bellowing voice of a policeman. “hey, there! i’ll report you!”

glancing back, hatch saw him standing in the middle of the street jotting down something in a note book. the chauffeur made a few uncomplimentary remarks about bluecoats in general, swished round a corner, and sped on. with a half smile of appreciation on his lips, hatch turned back to his unknown companion.

“if you will tell me where you are going,” he suggested, “i’ll have the chauffeur set you down.”

“it’s of no consequence,” replied the woman a little wearily. “i am going no place particularly — just riding about to collect my thoughts.”

a woman unattended, riding about in an automobile at fifteen minutes of eleven o’clock at night to collect her thoughts! and the chauffeur didn’t know he had a passenger! the reporter sat oblivious of the bumping, grinding, of the automobile, trying to consider this unexpected incident calmly.

“you are a reporter?” inquired the woman.

“yes,” hatch replied. “how did you guess it?”

“from seeing you rush out of a newspaper office in such a hurry at this time of night,” she replied. “something important, i dare say?”

“well, yes,” hatch agreed. “a jewel robbery at a ball. don’t know much about it yet. just got a police bulletin stating that mrs. windsor dillingham had been robbed of a necklace worth thirty thousand dollars at a big affair she is giving tonight.”

the inside of the cab was lighted brilliantly by the electric arc outside, and hatch had an opportunity of seeing the woman face to face at close range. she was pretty; she was young; and she was well dressed. from her shoulders she was enveloped in some loose cloak of dark material; but it was not drawn together at her throat, and her bare neck gleamed.

there being nothing whatever to say, hatch sat silently staring out of the window as the automobile whirled into commonwealth avenue and slowed up as it approached arden street.

“will you do me one favor, please?” asked the woman.

“yes, if i can,” was the reporter’s reply.

“allow me, please, to get out of the automobile on the side away from the curb, and be good enough to attract the attention of the chauffeur to yourself while i am doing it. here is a bill,” and she pressed something into hatch’s hand. “you may pay the chauffeur a tip for the passenger he didn’t know he had.”

hatch agreed in a dazed sort of way, and the automobile came to a stop. he stepped out on the curb, and slammed the door as the chauffeur leaped down from his seat. from the other side came an answering door slam, as if an echo.

five minutes later hatch joined detective mallory inside. at just that moment the detective was listening to the story of mrs. dillingham’s maid.

“there’s nothing missing but the necklace,” she explained; “so far, at least, as we have been able to find out. mrs. dillingham began dressing at about halfpast eight o’clock, and i assisted her as usual. i suppose it was halfpast nine when she finished. all that time the necklace was in the jewel box on her dressing table. it was the only article of jewelry in the box.

“well, the butler came up about halfpast nine o’clock for his final instructions, and mrs. dillingham went into the adjoining room to talk to him. it was not more than a minute later when she sent me down to the conservatory for a rose for her hair. she was still talking to him when i returned five minutes later. i put the rose in her hair, and she sent me into her dressing room for her necklace. when i looked into the jewel box, the necklace was gone. i told mrs. dillingham. the butler heard me. that’s all i know of it, except that mrs. dillingham went into hysterics and fainted, and i telephoned for a doctor.”

detective mallory regarded the girl coldly; hatch knew perfectly what was coming. “you are quite sure,” asked the detective, “that you did not take the necklace with you when you went down to the conservatory, and pass it to a confederate on the outside.”

the sudden pallor of the girl, her abject, cringing fright, answered the question to hatch’s satisfaction even before she opened her lips with a denial. hatch himself was about to ask a question, when a footman entered.

“mrs. dillingham will see you in her boudoir,” he announced.

from the lips of mrs. dillingham they heard identically the same story the maid had told. mrs. dillingham did not suspect anyone of her household.

for half an hour the detective interrogated her; then there came a rap at the door, and a woman entered.

“why, dora!” exclaimed mrs. dillingham

the young woman went straight to her, put her arms about her shoulders protectingly, then turned to glare defiantly at detective mallory and hutchinson hatch. the reporter gasped — it was the mysterious woman of the automobile. an exclamation was on his lips; but something in her eyes warned him, and he was silent.

when, on the following day, hutchinson hatch related the circumstances of the theft of mrs. dillingham’s necklace to professor augustus s. f. x. van dusen — the thinking machine — he did not mention the mysterious woman in the automobile. however curious those incidents in which he and she had figured were, they were inconsequential, and there was nothing to connect them in anyway with the problem in hand. the strange woman’s meeting with mrs. dillingham in the reporter’s presence had convinced him that she was an intimate friend.

“just what time was the theft discovered?” inquired the thinking machine.

“within a few minutes of halfpast nine.”

“at what time did most of the guests arrive?”

“between halfpast nine and ten.”

“then at halfpast nine,” continued the scientist, “there could not have been many persons there?”

“perhaps a dozen,” returned the reporter.

“and who were they?”

“their names, you mean? i don’t know.”

“well, find out,” directed the thinking machine crustily. “if the servants are removed from the case, and there were a dozen other persons in the house, common sense tells us to find out who and what they were. suppose, mr. hatch, you had attended that ball and stolen that necklace; what would have been your natural inclination afterward?”

hatch stared at him blankly for a minute, then smiled whimsically. “you mean how would i have tried to get away with it?” he asked.

“yes. when would you have left the place?”

“that’s rather hard to say,” hatch declared thoughtfully. “but i think i should either have gone before anybody else did, through fear of discovery, or else i should have been one of the last, through excess of caution.”

“then proceed along those lines,” instructed the thinking machine. “you might almost put that down as a law of criminology. it will enable you in the beginning, therefore, to narrow down the dozen or so guests to the first and last who left.”

deeply pondering this little interjection of psychology into a very material affair, hatch went his way. in the course of events he saw mrs. dillingham, who, out of consideration for her guests, flatly refused to give their names.

luckily for hatch, the butler didn’t feel that way about it at all. this was due partly to the fact that detective mallory had given him a miserable half-hour, and partly, perhaps, to the fact that the reporter oiled his greedy palm with a bill of two figures.

“to begin with,” said the reporter, “i want to know the names of the first dozen or so persons who arrived here that evening — i mean those who were here when you went up to speak with mrs. dillingham.”

“i might find out, sir. their cards were laid on the salver as they arrived, and that salver, i think, has remained undisturbed. therefore, the first dozen cards on it would give you the names you want.”

“now, that’s something like,” commented the reporter enthusiastically. “and do you remember any person who left the house rather early that evening?”

“no, sir,” was the reply. then suddenly there came a flash of remembrance across the stoical face. “but i remember that one gentleman arrived here twice. it was this way. mr. hawes campbell came in about eleven o’clock, and passed by without handing me a card. then i remembered that he had been here earlier and that i had his card. but i don’t recall that anyone went out, and i was at the door all evening except when i was up stairs talking to mrs. dillingham.”

on a bare chance, hatch went to find campbell. inquiry at his two clubs failed to find him, and finally hatch called at his home.

at the end of five minutes, perhaps, hatch caught the swish of skirts in the hallway, then the portieres were thrust aside, and — again he was face to face with the mysterious woman of the automobile.

“my brother isn’t here,” she said calmly, without the slightest sign of recognition. “can i do anything for you?”

her brother! then she was miss campbell, and mrs. dillingham had called her dora — dora campbell!

“well — er —” hatch faltered a little, “it was a personal matter i wanted to see him about.”

“i don’t know when he will return,” miss campbell announced.

hatch stared at her for a moment; he was making up his mind. at last he took the bit in his teeth. “we understand, miss campbell,” he said at last slowly and emphatically, “that your brother, hawes campbell has some information which might be of value in unraveling the mystery surrounding the theft of mrs. dillingham’s necklace.”

miss campbell dropped into a chair, and unconsciously hatch assumed the defensive. “mrs. dillingham is very much annoyed, as you must know,” miss campbell said, “about the publicity given to this affair; particularly as she is confident that the necklace will be returned within a short time. her only annoyance, beyond the wide publicity, as i said, is that it has not already been returned.”

“returned?” gasped hatch.

miss campbell shrugged her shoulders. “she knows,” she continued, “that the necklace is now in safe hands, that there is no danger of its being lost to her; but the situation is such that she cannot demand its return.”

“mrs. dillingham knows where the necklace is, then?” he asked.

“yes,” replied miss campbell.

“perhaps you know?”

“perhaps i do,” she responded readily. “i can assure you that mrs. dillingham is going to take the affair out of the hands of the police, because she knows her property is safe — as safe as if it was in your hands, for instance. it is only a question of time when it will be returned.”

“where is the necklace?” hatch demanded suddenly.

again miss campbell shrugged her shoulders.

“and what does your brother know about the affair?”

“i can’t answer that question, of course,” was the response.

“well, why did he go to mrs. dillingham’s early in the evening, then go away, and return about eleven o’clock?” insisted the reporter bluntly.

for the first time there came a change in miss campbell’s manner, a subtle, indefinable something which the reporter readily saw but to which he could attach no meaning.

“i can’t say more than i have said,” she replied after a moment. “believe me,” and there was a note of earnestness in her voice, “it would be far better for you to drop the matter, because otherwise you may be placed in-in a ridiculous position.”

and that was all — a threat, delicately veiled it is true, but a threat nevertheless. she arose and led the way to the door.

hatch didn’t realize the significance of that remark then, nor did it occur to him that the mysterious affair in the automobile had not been mentioned between them; for here was material, knotty, incoherent, inexplicable material, for the thinking machine, and there he took it. again he told the story; but this time all of it — every incident from the moment he hailed the automobile in front of his office on the night of the robbery until miss campbell closed the door.

“why didn’t you tell me all of it before?” demanded the thinking machine irritably.

“i couldn’t see that the affair in the automobile had any connection with the robbery,” explained the reporter.

“couldn’t see!” stormed the eminent man of science. “couldn’t see! every trivial happening on this whole round earth bears on every other happening, no matter how vast or how disconnected it may seem; the correlation of facts makes a perpetually unbroken chain. in other words, if mrs. leary hadn’t kept a cow, chicago would not have been destroyed by fire. couldn’t see!”

for an instant the thinking machine glared at him; and the change from petulant annoyance to deep abstraction, as that singular brain turned to the problem in hand, was almost visible. it was uncanny. then the scientist dropped back into his chair with eyes turned upward, and long slender fingers pressed tip to tip. ten minutes passed, twenty, thirty, and he turned suddenly to the reporter.

“what was the number of that automobile?” he demanded.

hatch grinned in sheer triumph. of all the questions he could have anticipated this was the most unlikely, and yet he had the number set down in his note book where it would ultimately become a voucher in his expense account. he consulted the book.

“number 869019,” he replied.

“now, find that automobile,” directed the thinking machine. “it is important that you do so at once.”

“you mean that the necklace —” hatch began breathlessly.

“when you bring the automobile here, i will produce the necklace,” declared the thinking machine emphatically.

hatch returned half a dozen hours later with troubled lines in his face.

“automobile no. 869019 has disappeared, evaporated into air,” he declared with some heat. “there was one that night, because i was in it, and the highway commission’s records show a private cab license granted to john kilrain under the number; but it has disappeared.”

“where is kilrain?” inquired the thinking machine.

“i didn’t see him; but i saw his wife,” explained the reporter. “she didn’t know anything about automobile no. 869019, or said she didn’t. she said his auto car was —”

“no. 610698,” interrupted the thinking machine. it was not a question; it was the statement as of one who knew.

hatch stared from the scientist to the note book where he had written down the number the woman gave him, and then he looked his utter astonishment.

“of course, that is the number,” continued the thinking machine, as if some one had disputed it. “it is past midnight now, and we won’t try to find it; but i’ll have it here tomorrow at noon. we shall see for ourselves how safely the necklace has been kept.”

detective mallory entered and glanced about inquiringly. he saw only the thinking machine and hutchinson hatch.

“i sent for you,” explained the scientist, “because in half an hour or so i shall either place the dillingham necklace in your hands, or turn over to you the man who knows where it is. you may use your own discretion as to whether or not you will prosecute. under all the circumstances, i believe the case is one for a sanatorium, rather than prison. in other words, the person who took the necklace is not wholly responsible.”

“who is it?” demanded the detective.

“you don’t happen to know all the facts in this case,” continued the thinking machine without heeding the question. “i got them all, only after mr. hatch, at my suggestion, had located the thief. originally i began where you left off. i believed you had eliminated the servants, and presumed there was not a burglary. ultimately this led to hawes campbell in a manner which is of no interest to you. then i got all the facts.

“when mr. hatch left his office to go to mrs. dillingham’s, he took an automobile which happened to be passing,” resumed the scientist. “it was a cab, no. 869019. inside that cab he found, much to his astonishment, a woman — a young woman in evening dress. she made the surprising statement that the chauffeur didn’t know she was there, and that she was not going anywhere — was merely riding around to collect her thoughts. and this was, please remember, about eleven o’clock at night. on its face this incident had no connection with the jewel theft; but by a singular chain of coincidences, subsequently developed, it seemed that mr. hatch had arrived at the solution of the mystery before he even knew the circumstances of the theft.”

detective mallory nodded doubtfully. “but how does that connect with the —” he began.

“subsequent developments establish a direct connection,” interrupted the thinking machine. “we have the woman in the automobile. we shall presume that she must have had some strong motive for leaving a house at that time of night and doing the apparently purposeless things that she did do. we don’t know this motive from these facts — we only know there was a motive.

“now when you and mr. hatch were talking to mrs. dillingham, a woman entered the room. mr. hatch recognized her immediately as the woman in the automobile. everything indicated that she was an intimate friend of mrs. dillingham’s. so we pass on to the point where mr. hatch found that hawes campbell arrived at the ball early, went away again, then returned after eleven o’clock. mr. hatch wanted to know why he left, and went to his home to inquire. campbell’s sister met him there. she was the woman he had met in the automobile. so we have campbell leaving the ball, immediately after the theft, say, and his sister running away from her home sometime between nine-thirty and eleven, and secreting herself in an automobile.

“why? i have said, mr. mallory, that imagination — the ability to bridge gaps temporarily — is the most essential part of the logical mind. now, if we imagine that campbell stole the necklace, that he went home, that his sister found it out, that there was some sort of scene which terminated in her flight with the necklace, we account for absolutely every incident preceding and following mr. hatch’s arrival at the dillingham place.

“i have made inquiries. the campbells are worth, not thousands, but millions. therefore, the question. why should hawes campbell steal a necklace? the answer, kleptomania. and again, it was known to the sister, who tried in her own manner to return the stolen property and avoid the scandal. when she was in the automobile, she was trying to collect her thoughts — trying to invent a way to return the necklace. it was the merest chance that mr. hatch happened to get into that particular vehicle.

“now, we come to the most difficult part of the problem,” and the thinking machine dropped back still further into the cavernous depths of his chair. “what would a frightened, perhaps hysterical, woman do with that necklace? from the fact that it has not been returned, we know that she didn’t venture into the house with it, and leave it casually in any one of a hundred places where it might have been discovered without danger to herself. yet everything indicates that she had it while in the cab. the obvious thing which suggests itself is that she hid it in the cab, intending to regain possession of it later and return it. now, that cab number was 869019. strangely enough, after mr. hatch left the cab it seems to have disappeared. the chauffeur, john kilrain, has another cab number now, 610698 — that is, auto cab no. 869019 was made to disappear by the simple act of turning the number board upside down, giving us 610698.”

“well, by george!” exclaimed detective mallory. no mere words would convey the reporter’s astonishment; he gasped.

“now,” continued the thinking machine after a moment, “there are two reasons, both good, why auto cab number 869019 should have disappeared. the vital one, it seems to me, is that kilrain discovered the necklace inside and kept it; the other is that he was threatened with arrest by the policeman who took his number for speeding, and to avoid a fine disguised the identity of his cab. there are one or two other possibilities; but if the necklace isn’t found in the automobile, i should advise, not arrest, but a close watch on kilrain, both at his home and in his intercourse with other chauffeurs at the various cab stands.”

there was a rap at the door, and martha appeared. “did you want an automobile, sir?”

“we’ll be right out,” returned the scientist.

and so it came about that the thinking machine, detective mallory, and hutchinson hatch searched the very vitals of auto cab no. 869019, temporarily masquerading as no. 610698, while kilrain stood by in perturbed amazement. at the end he was allowed to go.

“remember, please, what i advised you to do,” the thinking machine reminded detective mallory.

with eyes that were heavy with sleep hutchinson hatch crawled out of bed and answered the insistent ringing of his telephone. the crabbed voice of the thinking machine came over the wire, in a question.

“if miss campbell was so anxious to return the necklace that night, she couldn’t have done better, could she, than to have handed it to a reporter who was going to the house to investigate the robbery?”

“i don’t think so,” hatch replied wonderingly.

“did you have on your overcoat that night?”

“i had it with me.”

“suppose you go look in the pockets, and —”

hatch dropped the receiver, already inspired by the suggestion, and dragged his overcoat out of the closet. in the left hand lower pocket was a small package. he opened it with trembling fingers. there before his eyes lay the iridescent, gleaming bauble. it had been in his possession from an hour after it was stolen until this very instant. he rushed back to the telephone.

“i’ve got it!” he shouted.

“silly of me not to have thought of it in the first place,” came the querulous voice of the thinking machine. “good night.”