“Garnesk wasn’t there,” I protested.
“No,” my friend admitted, “he wasn’t there at the time, but he put me on the look-out for a left-handed sailor. I was very much impressed with his deductions about the man who stole Miss McLeod’s dog, and I determined to be on the look-out for a left-handed man. I also admit that I carefully watched everyone we met, especially the fishermen at Mallaig, to see if I could detect the sort of man I wanted. I was rewarded when we were pulled out to the Fiona by those two men of Fuller’s. One of them was red-headed, you remember? Well, that man was left-handed. It was very easy to observe that by the way he held his oar and generally handled things. Of course I was very bucked about it, so I paid very close attention to him. He wore a wedding-ring—ergo, he was married. It is not conclusive, of course, but a fairly safe guess when you’re playing at toy detectives. So when I found the knife I looked for some sign that it belonged to him, and found it. It was all quite simple.”
“I daresay it will be when you explain it, but you haven’t in the least explained it yet,” I pointed out. “How about the schoolmaster and all that, and what made you think the knife belonged to him.”
“Simply because he was very probably—working on the law of averages—the only left-handed man among the crew, and that knife belonged to a left-handed man.”
“But my dear old fellow,” I cried, “you don’t seriously mean to tell me that you can say whether a man is left-handed or not by looking at marks on the handle of his knife?”
“Not on the handle,” Dennis explained; “on the blade. Have you got a knife on you?”
I produced my pen-knife.
“I’ll trust you with it,” I declared confidently. “I’ve never held any secrets from you, Den.”
Dennis opened the knife and laid it in the palm of his hand. I stood still and watched him.
“You’ve sharpened pencils with this knife and the pencils have left their mark. If you hold the knife as you would when sharpening a pencil and look down on the blade there are no pencil marks visible. Now turn the knife over and you will find the marks on the other side of the blade.”