“That you have given me an antidote against old age,” was the ringing and unexpected reply, as the thoughtful, half-puzzled aspect of the old man yielded impulsively to a burst of his early enthusiasm.  “If we can get a good grip on the thread you speak of, and can work ourselves along by it, though it be by no more than an inch at a time, we shall yet make our way through this labyrinth of undoubted crime and earn for ourselves a triumph which will make some of these raw and inexperienced young fellows about us stare.  Sweetwater, coincidences are possible.  We run upon them every day.  But coincidence in crime! that should make work for a detective, and we are not afraid of work.  There’s my hand for my end of the business.”

“And here’s mine.”

Next minute the two heads were closer than ever together, and the business had begun.

XIII

TIME, CIRCUMSTANCE, AND A VILLAIN’S HEART

“Our first difficulty is this.  We must prove motive.  Now, I do not think it will be so very hard to show that this Brotherson cherished feelings of revenge towards Miss Challoner.  But I have to acknowledge right here and now that the most skillful and vigourous pumping of the janitor and such other tenants of the Hicks Street tenement as I have dared to approach, fails to show that he has ever held any communication with Mrs. Spotts, or even knew of her existence until her remarkable death attracted his attention.  I have spent all the afternoon over this, and with no result.  A complete break in the chain at the very start.”

“Humph! we will set that down, then, as so much against us.”

“The next, and this is a bitter pill too, is the almost insurmountable difficulty already recognised of determining how a man, without approaching his victim, could manage to inflict a mortal stab in her breast.  No cloak of complete invisibility has yet been found, even by the cleverest criminals.”

“True.  The problem is such as a nightmare offers.  For years my dreams have been haunted by a gnome who proposes just such puzzles.”

“But there’s an answer to everything, and I’m sure there’s an answer to this.  Remember his business.  He’s an inventor, with startling ideas.  So much I’ve seen for myself.  You may stretch probabilities a little in his case; and with this conceded, we may add by way of off-set to the difficulties you mention, coincidences of time and circumstance, and his villainous heart.  Oh, I know that I am prejudiced; but wait and see!  Miss Challoner was well rid of him even at the cost of her life.”

“She loved him.  Even her father believes that now.  Some lately discovered letters have come to light to prove that she was by no means so heart free as he supposed.  One of her friends, it seems, has also confided to him that once, while she and Miss Challoner were sitting together, she caught Miss Challoner in the act of scribbling capitals over a sheet of paper.  They were all B’s with the exception of here and there a neatly turned O, and when her friend twitted her with her fondness for these two letters, and suggested a pleasing monogram, Miss Challoner answered, ’O.  B. (transferring the letters, as you see) are the initials of the finest man in the world.’”