Karr said, “It doesn’t mean a damned thing to me. I fail to see why you are telling me about it.”
“Because,” Tragg went on patiently, “when any person walked across this beam of light without first lifting the telephone receiver, it caused a buzzer on the screen porch of the lower flat to sound. And that buzzer, Mr. Karr, was fastened to the side of the house so that it was directly below your bedroom window! ”
Karr placed his thin, wasted hand on the arm of the chair, gripped it so that the cords stood out plainly under the skin of the back of his hand. “Buzzer — under my window. Then that explains it.”
“Explains what?”
“That must have been what wakened me first, before I heard anything. I heard a peculiarly insistent sound which was like the buzzing of mosquitoes. It was high-pitched, distinctly audible, very irritating to a man of my nervous temperament. I kept listening, thinking at first it was a mosquito in the room, then realized that the sound was coming from outside of my window.”
“How long did it continue?” Tragg asked.
“Some little time. I don’t know how long it had been going before I woke up.”
“How long before you heard the shots?”
Karr said firmly, “There was only one shot.”
Tragg sighed. “I take it,” he said, “I am indebted for the other shot to the versatile mind of Mr. Mason.”