Willis had meanwhile spread a newspaper on the table and taken from his suitcase a small bottle of powdered lamp-black and a camel’s-hair brush. Laying the sketch on the newspaper he gently brushed some of the black powder over it, blowing off the surplus. To the satisfaction of both men, there showed up near the left bottom corner the distinct mark of a left thumb.
“Now the other side.”
Willis turned the paper and repeated the operation on the back. There he got prints of a left fore and second finger.
“Excellent, clear prints, those,” Willis commented, continuing: “And now I have something to tell you. While you were away I have been thinking over this thing, and I believe I’ve got an idea.”
Laroche looked interested, and the other went on slowly:
“There are two brandy-carrying lorries. Every night one of these lies at the distillery and the other at the clearing; one is being loaded and the other unloaded; and every day the two change places. Now we may take it that neither of those lorries is sent to any other place in the town, lest the brandy tanks might be discovered. For the same reason, they probably only make the one run mentioned per day. Is that right so far?”
“I should think so,” Laroche replied cautiously.
“Very well. Let us suppose these two lorries are Nos. 1 and 2. No. 1 goes to the distillery say every Monday, Wednesday and Friday, and returns on the other three days, while No. 2 does vice versa, one trip each day remember. And this goes on day after day, week after week, month after month. Now is it too much to assume that sooner or later someone is bound to notice this—some worker at the clearing or the distillery, some policeman on his beat, some clerk at a window over-looking the route? And if anyone notices it will he not wonder why it always happens that these two lorries go to this one place and to no other, while the syndicate has six lorries altogether trading into the town? And if this observer should mention his discovery to someone who could put two and two together, suspicion might be aroused, investigation undertaken, and presently the syndicate is up a tree. Now do you see what I’m getting at?”
Laroche had been listening eagerly, and now he made a sudden gesture.
“But of course!” he cried delightedly. “The changing of the numbers!”