The guard can take a two hour trick.

He can keep the fire going, and, if anything happens, he can warn the other fellows in turn." So, at nine o'clock, when the others turned in, Greg, the air rifle in one hand, paced softly up and down the cabin, watching, listening.
But nothing happened during Greg's watch.

At eleven he called Tom Reade to relieve him.
Just before midnight the same wailings as on the night before started in again.

Within sixty seconds all of the Grammar School boys were awake and listening.

The wailings continued, and soon came the same sepulchral warnings of death approaching.
"Queer that the racket doesn't bother us the way it did last night, isn't it ?" smiled Dick Prescott.
"It's awful enough!" shivered Hen Dutcher.