Harleston took another look around, saw no one, and calmly pocketed the envelope.  Then, after noting the number of the cab, No. 333, he gathered up the lines, whipped the ends about the box, and chirped to the horse to proceed.

The horse promptly obeyed; turned west on Massachusetts Avenue, and backed up to his accustomed stand in Dupont Circle as neatly as though his driver were directing him.

Harleston watched the proceeding from the corner of Eighteenth Street:  after which he resumed his way to his apartment in the Collingwood.

A sleepy elevator boy tried to put him off at the fourth floor, and he had some trouble in convincing the lad that the sixth was his floor.  In fact, Harleston’s mind being occupied with the recent affair, he would have let himself be put off at the fourth floor, if he had not happened to notice the large gilt numbers on the glass panel of the door opposite the elevator.  The bright light shining through this panel caught his eye, and he wondered indifferently that it should be burning at such an hour.

Subsequently he understood the light in No. 401; but then it was too late.  Had he been delayed ten seconds, or had he gotten off at the fourth floor, he would have—.  However, I anticipate; or rather I speculate on what would have happened under hypothetical conditions—­which is fatuous in the extreme; hypothetical conditions never are existent facts.

Harleston, having gained his apartment, leisurely removed from his pockets the handkerchief, the roses, and the envelope, and placed them on the library table.  With the same leisureliness, he removed his light top-coat and his hat and hung them in the closet.  Returning to the library, he chose a cigarette, tapped it on the back of his hand, struck a match, and carefully passed the flame across the tip.  After several puffs, taken with conscious deliberation, he sat down and took up the handkerchief.

This was Harleston’s way:  to delay deliberately the gratification of his curiosity, so as to keep it always under control.  An important letter—­where haste was not an essential—­was unopened for a while; his morning newspaper he would let lie untouched beside his plate for sufficiently long to check his natural inclination to glance hastily over the headlines of the first page.  In everything he tried by self-imposed curbs to teach himself poise and patience and a quiet mind.  He had been at it for years.  By now he had himself well in hand; though, being exceedingly impetuous by nature, he occasionally broke over.

His course in this instance was typical—­the more so, indeed, since he had broken over and lost his poise only that afternoon.  He wanted to know what was inside that blank envelope.  He was persuaded it contained that which would either solve the mystery of the cab, or would in itself lead on to a greater mystery.  In either event, a most interesting document lay within his reach—­and he took up the handkerchief.  Discipline!  The curb must be maintained.