after Markel told him to go to the front gate. The guard probably lied to Chet the first day we went to the mill-he never did phone the personnel department."
"Another thing," Joe pointed out. "Both men are more free to come and go than someone working in the plant."
There was silence while the Hardys concentrated on what their next move should be.
"No doubt about it," Frank said finally. "Everything seems to point toward the mill as the place to find the answers."
"And the only way to be sure," Joe added, "is to go and find out ourselves. How about tonight?"
Frank and Chet agreed, and the boys decided to wait until it was fairly dark. "I'll call Tony and see if he can go with us," Frank said. "We'll need his help."
Tony was eager to accompany the trio. "Sounds as if you're hitting pay dirt in the mystery," he remarked when Frank had brought him up to date.
"We hope so."
Later, Joe outlined a plan whereby they might ascertain if Peters was an accomplice of Docker
and Markel, and at the same time make it possible for them to get into the mill.
"Swell idea," Frank said approvingly. "Better brush up on your voice-disguising technique!"
Joe grinned. "Ill practice."
Just before supper Mr. Hardy phoned to say he would not be home until later that night.
"Making progress, Dad?" asked Frank, who had taken the call.
"Could be, son," the detective replied. "That's why I'll be delayed. Tell your mother and Gertrude not to worry."
"Okay. And, Dad-Joe and I will be doing some sleuthing tonight to try out a few new ideas we have."
"Fine. But watch your step!"
About eight-thirty that evening Chet and Tony pulled up to the Hardy home in the Queen.
Ken Blake went with the brothers to the door. "See you later, Ken," Frank said, and Joe added,
"I know you'd like to come along, but we don't want you taking any unnecessary risks."
The younger boy looked wistful. "I wish I could do something to help you fellows."
"There is a way you can help," Frank told him.
At that moment Mrs. Hardy and Aunt Gertrude came into the hall. Quickly Frank drew Ken
aside and whispered something to him.
CHAPTER XVII
Secret Signal
WITH rising excitement, Frank, Joe, Chet, and Tony drove off through the dusk toward the old
mill.
Chet came to a stop about one hundred yards from the beginning of the dirt road leading to the gatehouse. He and Tony jumped out. They waved to the Hardys, then disappeared into the
woods.
Joe took the wheel of the jalopy. "Now, part two of our plan. I hope it works."
The brothers quickly rode to the service station where they had been that morning. Joe parked
and hurried to the outdoor telephone booth nearby. From his pocket he took a slip of paper on
which Ken had jotted down the night telephone number of the Elekton gatehouse.
Joe dialed the number, then covered the mouthpiece with his handkerchief to muffle his voice.
A familiar voice answered, "Gatehouse. Markel speaking."
Joe said tersely, "Peters speaking. Something has gone wrong. Both of you meet me outside the Parker Building. Make it snappy!" Then he hung up.
When Joe returned to the Queen, Frank had turned it around and they were ready to go. They
sped back toward the mill and in about ten minutes had the jalopy parked out of sight in the
shadows of the trees where the dirt road joined the paved one.
The brothers, keeping out of sight among the trees, ran to join Chet and Tony who were waiting behind a large oak near the edge of the gatehouse grounds.
"It worked!" Tony reported excitedly. "About fifteen minutes ago the lights in the mill went out, and Markel and Docker left in a hurry."
"On foot?" Joe asked.
"Yes."
"Good. If they have to take a bus or cab to town, it'll give us more time," Frank said.
Tony and Chet were given instructions about keeping watch outside while the Hardys inspected
the mill.
The brothers explained where the Queen was parked, in case trouble should arise and their
friends had to go for help.
Frank and Joe approached the mill cautiously. It was dark now, but they did not use flashlights.
Though confident that the gatehouse was deserted, they did not wish to take any chances. As
they neared the building the Hardys could see that the shutters were tightly closed. Over the
sound of the wind in the trees came the rumble of the turning mill wheel.
The Hardys headed for the door. They had just mounted the steps when the rumbling sound of
the wheel ceased.
In the silence both boys looked around, perplexed. 'I thought it had been fixed," Joe whispered.
"Seemed okay the other day."
"Yes. But last time we were here at night the wheel stopped when we were about this distance away from it," Frank observed.
Thoughtfully the boys stepped back from the mill entrance to a point where they could see the
wheel.
They stood peering at it through the darkness. Suddenly, with a dull rumble, it started to turn again!
Mystified, the Hardys advanced toward the gatehouse and stopped at the entrance. In a short
while the wheel stopped.
"Hm!" Joe murmured. "Just like one of those electric-eye doors."
"Exactly!" Frank exclaimed, snapping his fingers. "I'll bet the wheel's not broken-it's been rigged up as a warning signal to be used at night!
When someone approaches the mill, the path of the invisible beam is broken and the wheel
stops. The lack of noise is enough for anyone inside to notice, and also, the lights would go out because the generator is powered by the wheel."
The Hardys went on a quick search for the origin of the light beam. Frank was first to discover that it was camouflaged in the flour-barrel ivy planter. Beneath a thin covering of earth, and barely concealed, were the heavy batteries, wired in parallel, which produced the current
necessary to operate the light source for the electric eye.
The stopping and starting of the wheel was further explained when Frank found, screened by a
bushy shrub, a small post with a tiny glass mirror fastened on its side.
"That's the complete secret of the signal!" he exclaimed. "This is one of the mirrors a photo-electric cell system would use. With several of these hidden mirrors, they've made a light-ring around the mill so an intruder from any side would break the beam. The barrel that contains
the battery power also contains the eye that completes the circuit."
"I'll bet Markel and Docker rigged this up," Joe said excitedly. "Which means there must be something in the mill they want very badly to keep secret! We must find a way inside!"
The Hardys did not pull the wires off the battery connection, since they might have need of the warning system. Quietly and quickly the brothers made a circuit of the mill, trying doors and
first-floor windows, in hopes of finding one unlocked. But none was.
"We can't break in," Joe muttered. Both boys were aware that time was precious-the men might return shortly. The young sleuths made another circle of the mill. This time they paused to stare at the huge wheel, which was turning once more.
"Look!" Joe whispered tensely, pointing to an open window-shaped space above the wheel.
"It's our only chance to get inside," Frank stated. "We'll try climbing up."
The Hardys realized it would not be easy to reach the opening. Had there been a walkway on