After lunch everyone but Chet was ready to relax in the sun. Normally he was the first one to
suggest a period of rest, even a nap, but now his new project was uppermost in his mind.
'Let's start to collect the specimens for my microscope,' he urged his friends.
The Hardys groaned good-naturedly at Chet's enthusiasm, but readily agreed.
'We'll need some exercise to work off that meal.' Frank grinned.
The girls packed the food wrappings in the hamper. Then, single file, the group walked
downstream, paying careful attention to the rocks and vegetation. Chet picked up several rocks and leaves, but discarded them as being too common.
'Are you looking for something from the Stone Age?' Joe quipped. 'Maybe a prehistoric fossil?'
'Wouldn't you be surprised if I found one?' Chet retorted.
They followed a bend in the river and came to a small cove with a rocky, shelving beach. Here
the willow trees did not grow so thickly. The shoreline curved gently around to the right before it came to a halt in a sandy strip along the riverbank.
'What a nice spot,' said Callie. 'We'll have to come here again and wear our swim suits.'
'Look!' cried Iola. 'What's that?'
She pointed to a dark opening beneath a rocky ledge which bordered the beach.
'A cave!' exclaimed Joe and Frank together.
Intrigued, the five hurried along the beach for a closer look. Eagerly the Hardys and Chet
peered inside the entrance. The interior was damp, and the cave's walls were covered with
green growth.
'This'll be a perfect spot to look for specimens,' Chet said. 'Let's go in!'
The boys entered the cave. The girls, however, decided to stay outside.
'Too spooky-and crowded!' Callie declared. 'Iola and I will sun ourselves while you boys explore.'
The Hardys and Chet could just about stand up in the low-ceilinged cave. Frank turned on his
pocket flashlight and pointed to an unusual yellow-green fungus on the right side of the cave.
'Here's a good sample of lichens, Chet.'
Soon the boys were busy scraping various lichens off the rocks. Gradually they moved deeper
into the cave. Frank halted in front of a pile of rocks at the rear.
'There ought to be some interesting specimens behind these stones,' he said. 'They look loose enough to
move.'
Together, the three boys rolled some of the rocks to one side. To their great surprise, the
stones had concealed another dark hole.
'Hey! This looks like a tunnel!'
Excitedly Joe poked his flashlight into the opening. In its beam they could see that the hole
appeared to extend into the side of the bank.
'Let's see where the tunnel goes!' Joe urged.
'Okay,' Frank agreed eagerly. 'We'll have to move more of these rocks before we can climb through. I wonder who put them here and why.'
Rapidly the boys pushed rocks aside until the narrow tunnel entrance was completely exposed.
Joe crawled in first, then Frank.
Chet tried to squeeze his bulky form through the space but quickly backed out. 'It's too tight for me,' he groaned. 'I'll stay here and collect more specimens. Anyhow, I'll bet some animal made the tunnel and it doesn't lead anywhere.'
'I'm sure no animal did this,' Joe called back, aiming his flashlight at the earthen walls of the tunnel. 'Look how hard-packed the sides are-as if dug out by a shovel,'
Frank was of the same opinion. He pointed to rough-hewn wooden stakes placed at intervals
along the sides and across the ceiling. 'I wonder who put those supports here-and when.'
The Hardys crawled ahead carefully. There was just room in the passageway for a normal-sized
person to get through.
Presently Joe called back to his brother, 'Look ahead! I can see a sharp bend to the right. Let's keep going.'
Frank was about to reply when the brothers were startled by a girl's scream from outside.
'That's Callie!' Frank exclaimed. 'Something's wrong!'
CHAPTER VII
Sleuthing by Microscope
FRANK and Joe scrambled through the tunnel and out of the cave. They found Chet and the girls
staring at an arrow embedded in the sandy beach.
'It-it almost hit us,' Iola quavered. Callie, who was white-faced with fear, nodded.
Joe was furious. 'Whoever shot it shouldn't be allowed to use such a dangerous weapon!' he burst out.
'That's a hunting arrow-it could have caused serious injury.'
Chet gulped. 'M-maybe the Indians haven't left here, after all,' he said, trying to hide his nervousness.
Joe turned to dash off into the woods to search for the bowman.
'Wait!' Frank called. He had pulled the arrow from the sand. 'This was done deliberately,' he announced grimly, holding the arrow up for all of them to see. Attached to the shaft just below the feathers was a tiny piece of paper. It had been fastened on with adhesive tape.
Frank unrolled the paper and read the printed message aloud: ' 'Danger. Hardys beware.' '
Chet and the girls shuddered and looked around fearfully, as though they expected to see the
bowman behind them. 'You boys are involved in a new mystery!' Callie exclaimed. 'Your own or your father's?'
Frank and Joe exchanged glances. It certainly seemed as though they were involved, but they
had no way of knowing which case. Did it involve the counterfeit money? Or was it the case
their father could not divulge?
'A warning did come to Dad,' Frank admitted. 'This one obviously was meant for Joe and me.
Whoever shot the arrow trailed us here.'
Joe frowned. 'I wonder if the same person sent both warnings.'
'I still think Ken Blake could give us a clue,' Frank said. 'But we must remember that anybody could have taken the bike from the storage place under the mill.'
Frank pocketed the latest warning, then the five searched quickly for any lead to the bowman.
They found none. When the group returned to the beach, Joe looked at the sky. 'We're in for a storm-and not one of us has a raincoat.' The bright summer sun had disappeared behind
towering banks of cumulus clouds. There were rumbles of heavy thunder, followed by vivid
flashes of lightning. The air had become humid and oppressive.
'Let's get out of here!' Chet urged. 'This isn't a picnic any more!'
The young people hastened through the woods and up the road to Chet's jalopy. As they drove
off, rain began coming down in torrents. The sky grew blacker.
Callie shivered. 'It seems so sinister-after that awful arrow.'
Chet dropped his sister off at the Morton farm and at the same time picked up his new
microscope. He begged to try out the instrument on both warning notes and the Hardys
smilingly agreed, although they had an up-to-date model of their own.
By the time they had said good-by to Callie at her house, and Chet had driven the Queen into
the Hardys' driveway, the storm had ended. The sun shone brightly again.
Immediately the three boys went to the laboratory over the garage. Here Frank carefully dusted the arrow and the second warning note for prints. He blew the powder away, and Joe and Chet
looked over his shoulder as he peered through the magnifying glass.
'Nothing. Same as the warning to Dad. The person no doubt wore gloves.'
'Now to compare this paper to the first note,' Joe said.
'Right,' his brother agreed, 'You have the combination to the cabinet in Dad's study. Chet and I will rig up his
