Bart opened the door and peered cautiously out into the hall.

"Coast is clear," he whispered over his shoulder.

In another moment they were outside the room. Along the corridor they skurried like cats, their feet making no noise on the floor.

Frank was still entirely unaware of their destination, but, as they had not taken their shoes, he knew they were not to leave the building.

Frank cared little where they went, but he realized Hodge was leading the way to a remote part of the building, where the rooms were not entirely taken, as the academy was not full of students.

All at once, Bart sent a peculiar hiss down the corridor, and it was answered by a similar sound.

A moment later they scudded past a fellow who was hugging in a shadow where the lights did not reach.

"Who's that?" whispered Frank.

"That's the sentinel," replied Bart.

Then they came to the door of a certain room, on which Hodge knocked in a peculiar manner.