"The essential part of this matter," said Nick, "was this door which appeared to open and close of itself.
"I saw that at a glance, and made a secret investigation. It is done by electricity.
"There's a magnet in the casing which is powerful enough to swing the door to, after which the same magnet pushes this little bolt—which looks like an ordinary screw—into position, and that holds the door, but not very steadily.
"You may say that this should have given me the criminal at once, but it didn't.
"You see, this electro-magnet works whenever a current is turned into the wires. Horace was clever enough to have the wires lead all over the house.
"A connection with the electric light wires, furnishing the current, can be made in almost every room in the house.
"Of course, I suspected Horace at once, because his room was directly overhead. In fact, the two are connected, as you see, by a ventilator in the form of a pipe with a grated opening in each room.
"The grating here, you see, is open."
"But, bless me," exclaimed Judge Lorrimer, "no thief could come through such a place. Why, it isn't six inches square."
"Step in here a minute and see," said Nick, and then he called out: