answered, pushing
up a trap door that led to a small square platform on the roof. "It's
the motor sound the girls heard and that scared them so."

"It is, for a fact!" cried Teddy in a joyful whisper. "And it's coming
right near, fellows, too."

"It's an aeroplane all right," said Ferd, with conviction. "Nothing else
ever made a noise like that."

"Say, what are you doing up there?" a girl's voice hailed them from the
bottom of the steps, and Chet thought he recognized it as Billie's. "Are
you walking in your sleep or have you gone crazy? Come down here quick,
we need you."

"Keep still," Chet yelled back. "We're looking for your aeroplane ghost.
Can't you hear it?"

"Yes. But, oh, Chet," Billie's voice was tremulous, "the piano is playing
itself again. Won't you come down? We're afraid to stay here all alone."

"Great Scott! all the spirits are roaming at once," cried Teddy,
straining his eyes to see through the darkness as the humming of the
motor came nearer.

"There, isn't that it?" cried Ferd, pointing eagerly through the trees
toward a little patch of sky, palely illumined with stars.

"I think I saw it," said Chet, rubbing his eyes impatiently. "It's so
confoundedly dark--"

"Oh, won't you please come down?" wailed Billie's voice from the
spooky depths of the attic. "I'll die of fright if I have to stay here
another minute."

This appeal moved the boys, and they began reluctantly to descend the
ladder, keeping their eyes all the time on the pale patch of sky.

"Where are the others?" asked Teddy, as he reached Billie's side.

"They're down looking for the ghost," answered Billie, as she ran down
the stairs in front of them. "They sent me to get you boys, and I found
you gone. Mrs. Gilligan," she added, with a hysterical giggle, "has the
broom and Laura has the poker."

"Maybe we'd better stop on the way and gather up a few bedposts,"
suggested Ferd, as they took the last flight of stairs on a run and
landed in the lower hall.

"Hello, did you find anything?" sang out Chet, as the girls, looking
scared but valiant, came out to meet them. "Where's Mrs. Gilligan?"

"Inside," said Violet. "There isn't a thing to be seen any more than
there was the other night. I'm absolutely positive now that it must
be a ghost."

"Well, if it is, he's got a sense of humor," said Mrs. Gilligan, rising
from her knees where she had been peering into the corner behind the
piano. "I've heard of all sorts of spirits,