slipping back into the hallway , he looked for a way out , and followed a breath of air to the entryway : a very indifferent sentry drowsed in a corner , and laurence was past him and had a foot in the gardens when a great roaring came from above , a sound at once familiar and bone-rattling , though he had not heard it since the battle of the nile : a dragon , overhead , and the lights of all the house flared at his back .