A cab was by the curb, its horse lying prostrate on the asphalt, its box vacant of driver.

Harleston stopped.  What had he here!  Then he looked about for a policeman.  Of course, none was in sight.  Policemen never are in sight on Massachusetts Avenue.

As a general rule, Harleston was not inquisitive as to things that did not concern him—­especially at one o’clock in the morning; but the waiting cab, the deserted box, the recumbent horse in the shafts excited his curiosity.

The cab, probably, was from the stand in Dupont Circle; and the cabby likely was asleep inside the cab, with a bit too much rum aboard.  Nevertheless, the matter was worth a step into Eighteenth Street and a few seconds’ time.  It might yield only a drunken driver’s mutterings at being disturbed; it might yield much of profit.  And the longer Harleston looked the more he was impelled to investigate.  Finally curiosity prevailed.

The door of the cab was closed and he looked inside.

The cab was empty.

As he opened the door, the sleeping horse came suddenly to life; with a snort it struggled to its feet, then looked around apologetically at Harleston, as though begging to be excused for having been caught in a most reprehensible act for a cab horse.

“That’s all right, old boy,” Harleston smiled.  “You doubtless are in need of all the sleep you can get.  Now, if you’ll be good enough to stand still, we’ll have a look at the interior of your appendix.”

The light from the street lamps penetrated but faintly inside the cab, so Harleston, being averse to lighting a match save for an instant at the end of the search, was forced to grope in semi-darkness.

On the cushion of the seat was a light lap spread, part of the equipment of the cab.  The pockets on the doors yielded nothing.  He turned up the cushion and felt under it:  nothing.  On the floor, however, was a woman’s handkerchief, filmy and small, and without the least odour clinging to it.

“Strange!” Harleston muttered.  “They are always covered with perfume.”

Moreover, while a very expensive handkerchief, it was without initial—­which also was most unusual.

He put the bit of lace into his coat and went on with the search: 

Three American Beauty roses, somewhat crushed and broken, were in the far corner.  From certain abrasions in the stems, he concluded that they had been torn, or loosed, from a woman’s corsage.

He felt again—­then he struck a match, leaning well inside the cab so as to hide the light as much as possible.

The momentary flare disclosed a square envelope standing on edge and close in against the seat.  Extinguishing the match, he caught it up.

It was of white linen of superior quality, without superscription, and sealed; the contents were very light—­a single sheet of paper, likely.

The handkerchief, the crushed roses, the unaddressed, sealed envelope—­the horse, the empty and deserted cab, standing before a vacant lot, at one o’clock in the morning!  Surely any one of them was enough to stir the imagination; together they were a tantalizing mystery, calling for solution and beckoning one on.