It was a puzzling one, too. Every policeman who had been on night duty in Lytton Avenue for months was closely examined. Once or twice a night the doors of the house had been tried without effect. Nobody had ever been seen to come away or enter. No suspicious characters had been seen loafing about. Not one of the officers had ever seen a light in the place.
"I'll go and look at the gas meter," said Prout.
He was an efficient officer in his way, only, like most members of the force, he lacked imagination. Give him something to work on, and there was not a more efficient detective in New Scotland Yard. But there was no clue here, so he had to fall back on the old familiar methods.
Here was the gas meter under the stairs as usual. Behind it was the grimy, dirty card, which showed no entry for years. It was marked "taken 5 Feb.," in other words the meter had been read the day the owner had disappeared. By reading the index Prout saw that a hundred odd cubic feet of gas had been used since.
Here was something to go upon. Beyond doubt that gas had been used lately. Prout made a careful examination of the burners, sniffing and blowing at all of them. He found out one thing, only the burners in the hall and the bedroom where the murdered man had been found had been used for a long time. In a bedroom at the top of the house was a paraffin lamp with quite a new wick in it. With a stump of pencil Prout made a rapid calculation on the wall-paper.
"Lamp used by murderer waiting for his victim," he deduced. "Did not want any more light than was necessary, so probably lay low in a back room. When the hour for the victim came, lighted the hall gas so as not to look suspicious. Then why the dickens didn't the officer on duty notice it?"
"Because it wasn't there when he passed, Prout," said a quick voice that caused the detective to turn with a start. "There was a confederate, of course. Nothing easier than for the confederate to listen for the officer's footsteps and put out the gas till he had gone by. Other people didn't matter."
"Right as usual, Mr. Lawrence," said Prout, beaming approvingly on the great novelist. "Why don't you come and join the force?"
Lawrence modestly disclaimed the compliment. As a strong romantic writer he found a fascination in crimes of this kind; indeed, he boasted that practically all his living dramas were founded on life. He had a wonderful faculty for tracing the motive of a crime. Many a useful hint had he given to Scotland Yard.
"What's the theory here, sir?" Prout asked respectfully.