“ No. 27,” said the inspector.

“It is here; unlock the door, Antoine.”

The turnkey obeyed, and the inspector gazed curiously into the chamber of the “mad abbé,” as the prisoner was usually called.

In the centre of the cell, in a circle traced with a fragment of plaster detached from the wall, sat a man whose tattered garments scarcely covered him. He was drawing in this circle geometrical lines, and seemed as much absorbed in his problem as Archimedes was when the soldier of Marcellus slew him.

He did not move at the sound of the door, and continued his calculations until the flash of the torches lighted up with an unwonted glare the sombre walls of his cell; then, raising his head, he perceived with astonishment the number of persons present. He hastily seized the coverlet of his bed, and wrapped it round him.