each door-post, about three feet from the ground, and I'm going to
stretch this piece of black cotton between them. No one will see it,
and if they go through the door, the thread will simply draw out of one
of the slits without their noticing it, and we shall see that it's been
disturbed. Jack Vance says that when he's been out shooting with his
guv'nor he's seen the keeper put them across the paths in a wood to find
out if poachers have been up them. Now unlock the door, and let's go
inside."

In front of the bench, where the ground had been much trodden, there was
a great deal of loose dust. Diggory went down on his hands and knees,
and producing an old clothes-brush from his pocket, swept about a square
yard of the ground until the dust lay in a perfectly smooth surface.

"There," he said, rising to his feet again; "we'll do this the last
thing every night, and any morning if we find the cotton gone we must
look here for footprints, and then we ought to be able to tell if it's
a man or a boy."

"Don't you think we ought to tell Blake about that man you saw?" asked
Acton, as they walked back to the schoolroom.

"Well, I don't see how we can," answered Diggory. "The first thing
he'll ask will be,' Who saw him?' I shall say, 'I did;' and then he'll
want to know how I saw the playground door from my bedroom window, which
looks out on the road; and then the fat'll be in the fire, and it'll all
come out about that supper."

Regularly every evening, as soon as supper was over, the two boys stole
down into the playground to set their trap; but when morning came there