“Did you post that letter yourself?”

The nobleman’s reply was interrupted by his secretary, who broke in with some heat.

“His Grace is not in the habit of posting letters himself,” said he. “This letter was laid with others upon the study table, and I myself put them in the post-bag.”

“You are sure this one was among them?”

“Yes, I observed it.”

“How many letters did your Grace write that day?”

“Twenty or thirty. I have a large correspondence. But surely this is somewhat irrelevant?”

“Not entirely,” said Holmes.

“For my own part,” the Duke continued, “I have advised the police to turn their attention to the south of France. I have already said that I do not believe that the Duchess would encourage so monstrous an action, but the lad had the most wrong-headed opinions, and it is possible that he may have fled to her, aided and abetted by this German. I think, Dr. Huxtable, that we will now return to the Hall.”

I could see that there were other questions which Holmes would have wished to put, but the nobleman’s abrupt manner showed that the interview was at an end. It was evident that to his intensely aristocratic nature this discussion of his intimate family affairs with a stranger was most abhorrent, and that he feared lest every fresh question would throw a fiercer light into the discreetly shadowed corners of his ducal history.

When the nobleman and his secretary had left, my friend flung himself at once with characteristic eagerness into the investigation.

The boy’s chamber was carefully examined, and yielded nothing save the absolute conviction that it was only through the window that he could have escaped. The German master’s room and effects gave no further clue. In his case a trailer of ivy had given way under his weight, and we saw by the light of a lantern the mark on the lawn where his heels had come down. That one dint in the short, green grass was the only material witness left of this inexplicable nocturnal flight.

Sherlock Holmes left the house alone, and only returned after eleven. He had obtained a large ordnance map of the neighbourhood, and this he brought into my room, where he laid it out on the bed, and, having balanced the lamp in the middle of it, he began to smoke over it, and occasionally to point out objects of interest with the reeking amber of his pipe.

“This case grows upon me, Watson,” said he. “There are decidedly some points of interest in connection with it. In this early stage, I want you to realize those geographical features which may have a good deal to do with our investigation.

“Look at this map. This dark square is the Priory School. I’ll put a pin in it. Now, this line is the main road. You see that it runs east and west past the school, and you see also that there is no side road for a mile either way. If these two folk passed away by road, it was this road.”

“Exactly.”