‘Look here, Watson,’ he said, when the cloth was cleared; ‘just sit down in this chair and let me preach to you for a little. I don’t quite know what to do, and I should value your advice. Light a cigar, and let me expound.’

‘Pray do so.’

‘Well, now, in considering this case there are two points about young McCarthy’s narrative which struck us both instantly, although they impressed me in his favour and you against him. One was the fact that his father should, according to his account, cry “Cooee!” before seeing him. The other was his singular dying reference to a rat. He mumbled several words, you understand, but that was all that caught the son’s ear. Now from this double point our research must commence, and we will begin it by presuming that what the lad says is absolutely true.’

‘What of this “Cooee!” then?’

‘Well, obviously it could not have been meant for the son. The son, as far as he knew, was in Bristol. It was mere chance that he was within earshot. The “Cooee!” was meant to attract the attention of whoever it was that he had the appointment with. But “Cooee” is a distinctly Australian cry, and one which is used between Australians. There is a strong presumption that the person whom McCarthy expected to meet at Boscombe Pool was someone who had been in Australia.’

‘What of the rat, then?’

Sherlock Holmes took a folded paper from his pocket and flattened it out on the table. ‘This is a map of the Colony of Victoria,’ he said. ‘I wired to Bristol for it last night.’ He put his hand over part of the map. ‘What do you read?’ he asked.

‘Arat,’ I read.

‘And now?’ he raised his hand.

‘Ballarat.’

‘Quite so. That was the word the man uttered, and of which his son only caught the last two syllables. He was trying to utter the name of his murderer. So-and-so of Ballarat.’

‘It is wonderful!’ I exclaimed.

‘It is obvious. And now, you see, I had narrowed the field down considerably. The possession of a grey garment was a third point which, granting the son’s statement to be correct, was a certainty. We have come now out of mere vagueness to the definite conception of an Australian from Ballarat with a grey cloak.’

‘Certainly.’

‘And one who was at home in the district, for the Pool can only be approached by the farm or by the estate, where strangers could hardly wander.’

‘Quite so.’

‘Then comes our expedition of to-day. By an examination of the ground I gained the trifling details which I gave to that imbecile Lestrade, as to the personality of the criminal.’

‘But how did you gain them?’

‘You know my method. It is founded upon the observance of trifles.’