"So far the crime is commonplace and vulgar enough," Jack Masefield suggested. "Scores of these things happen in London every year. Some are found out, but some remain mysteries to the end of time; but this particular crime seems to be peculiarly terrible. First of all, London for some time has been doubly attentive to the yellow-faced posters. No greater advertising circular has ever appealed to the public. Nostalgo is a personality about as great as some of our leading actors. Still, nobody has really regarded Nostalgo as a living force, and I find him dead on the pavement here right in front of one of his own posters. Is that coincidence or an amazing happening?"

"Both, I should say, sir," Bates replied. "An amazing happening in any case. But to find the man dead in front of one of his own posters may be no more than a coincidence. You see, there are so many Nostalgo posters about."

But Jack was loth to give up his point.

"I admit that," he said; "but the particular poster we find up is a fresh one. It was more or less shot-marked, as I pointed out to you; it was marked much as the body of the dead man was marked. If you remember, I suggested examining the poster by means of a magnifying glass, in the hope of finding some kind of printer's trade-mark, and we come back here for that purpose. We find the poster pasted over with a commonplace advertisement of somebody's mustard. Surely that is not coincidence. For some reason or other the poster was covered by design. It is not the habit of the bill-poster to go about the work at midnight."

"Ah, there you are not altogether correct, sir," Bates exclaimed. He felt that he was on pretty safe ground now. "The working bill-poster is not tied to time. He has a certain amount of work to do, and he does it pretty well when he pleases. Sometimes they have to work very late. For instance, a stock piece put up at a theatre may prove a draw, and the management desire to keep it going for a time. Then there is work late at night for some firm of the paste-pot."

"Quite so, inspector; but does that apply to the harmless, necessary mustard advertisement?"

"Not directly, perhaps. But suppose there had been a sudden rush of new and urgent work, the routine would have fallen behind. Please understand that the bill-poster does not career round in a casual way, sticking up a poster just where it suits his fancy. All these hoardings are rented, and big advertisers contract to have so many sheets displayed every week; in fact, it is a most desultory business. Depend upon it, the bill-poster who so lately posted up that alluring mustard tin had nothing to do with the business."

It was all so logical and conclusive that Jack was compelled to drop further argument. At the same time, it seemed rather foolish to stand there doing nothing.

"Look here," he said, struck by a sudden idea; "why not pull that mustard poster down, and get at the real source of the truth. The paper is still wet, and I dare say we might find a ladder behind the hoarding. Let us pull it down, and take the whole thing to the police-station and examine it at our leisure."

There was no objection to this, as Bates was bound to admit. It was a very easy matter to find a way behind the hoarding and secure the firmest of many ladders. A short one was sufficient for the purpose, and very soon the great sheet that contained the mustard advertisement was pulled off the wooden hoarding and lay in a heap on the pavement. In the place of it, fresh and strong, was the yellow face of Nostalgo. Jack took the inspector's lamp and regarded the poster carefully by the magnifying glass. But there was no imprint to be seen, nothing to lead to the identity of the firm who printed the placard.