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 identify him: if he is known, the police have no need of the bloodhound.

"To return to our present case," Thorndyke resumed, after a pause; "we have employed certain agents—the hounds—with whom we are not en rapport, as the spiritualists would say; and we have no 'medium.' The hound possesses a special sense—the olfactory—which in man is quite rudimentary. He thinks, so to speak, in terms of smell, and his thoughts are untranslatable to beings in whom the sense of smell is undeveloped. We have presented to the hound a knife, and he discovers in it certain odorous properties; he discovers similar or related odorous properties in a tract of land and a human individual—Ellis. We cannot verify his discoveries or ascertain their nature. What remains? All that we can say is that there appears to exist some odorous relation between the knife and the man Ellis. But until we can ascertain the nature of that relation, we cannot estimate its evidential value or bearing. All the other 'evidence' is the product of your imagination and that of the general. There is, at present, no case against Ellis."

"He must have been pretty close to the place when the murder happened," said the inspector.

"So, probably, were many other people," answered Thorndyke; "but had he time to wash and change? Because he would have needed it."