The waterfall of Lake Romano was still some little distance off, and, as the wind was blowing toward it, only a faint roar of the falling water came to the ears of the Bobbsey twins, and the others on the houseboat.

"Oh, papa!" exclaimed Nan. "May we go close up and see the cataract?"

"Yes," said Mr. Bobbsey. "I intended to give you a good view of the waterfall. We shall spend a day or so here, as it is a great curiosity. There is one place where you can walk right behind the falls."

"Behind it!" cried Harry. "I don't understand how that can be, uncle."

"You'll see to-morrow, when we visit them," said the twins' father.
"And there are some oddly-marked stones to be picked up, too, Bert.
They will do for your collection."

"Fine!" Bert exclaimed. "Say, this has been a dandy trip all right!"

"It isn't ended yet, is it, Dorothy?" asked Nan.

"No, indeed," replied the seashore cousin, with a smile.

"And we haven't solved the mystery," said Bert in a low voice to
Harry. "But we will to-night, all right."

"We sure will," agreed the boy from the country.

The Bobbsey twins stayed up rather later that night than usual. Mr. Bobbsey did not find a good anchorage for the boat for some time, as he wanted to get in a safe place. It looked as though there might be a storm before morning, and he did not want to drift away again. Then, too, he wanted to get nearer to the waterfall, so they could reach it early the next morning and look at it more closely.

So the motor was kept in action by Captain White until after supper, and finally the Bluebird came to rest not far from the waterfall. Then Bert and Nan, with Dorothy and Harry were so interested in listening to Mr. Bobbsey tell stories about waterfalls, and what caused them, that the older twins and their cousins did not get to bed until nearly ten o'clock, whereas nine was the usual hour.

Of course Flossie and Freddie "turned in," as sailors say, about eight o'clock, for their little eyes would not stay open any longer.

"We'll wake up as soon as my father and mother are asleep," said Bert to Harry, as they went to their rooms, which were adjoining ones. "Then we'll take turns watching that closet."

"Sure," agreed Harry. "Whoever wakes up first, will call the other."

To this Bert agreed, but the truth of it was that neither of them awakened until morning. Whether it was that they were too tired, or slept later than usual, they could not tell. But it was broad daylight, when they sat up in their beds, or "bunks," as beds are called on ships.

"I thought you were going to call me," said Bert to his cousin.

"And I thought you were going to call ME," laughed the boy from the country.

Then they both laughed, for it was a good joke on each of them.

"Never mind," spoke Bert, as he got up and dressed. "We'll try it again to-night."

"Try what?" asked Nan from the next room, for she could hear her brother speak. "If you boys try to play any tricks on us girls—-"

"Don't worry," broke in Harry. "The secret isn't about you."

"I think you're real mean not to tell us!" called Dorothy, from her room. "Nan and I are going to have a marshmallow roast, when we go on shore near the waterfall, and we won't give you boys a single one, will we, Nan?"

"Not a one!" cried Bert's sister.

"Will you give me one—whatever it is?" asked Freddie from the room where his mother was dressing him.

"And me, too?" added Flossie, for she always wanted to share in her little twin brother's fun.

"Yes, you may have some, but not Bert and Harry," went on Nan, though she knew when the time came, that she would share her treat with her brother and cousin.

"Well, I didn't hear any noises last night," said Mr. Bobbsey to his wife at the breakfast table.

"Nor I," said she. But when Dinah came in with a platter of ham and eggs, there was such a funny look on the cook's face that Mrs. Bobbsey asked:

"Aren't you well, Dinah?"

"Oh, yes'm, I'se well enough," the fat cook answered. "But dey shuah is suffin strange gwine on abo'd dish yeah boat."

"What's the matter now?" asked Mr. Bobbsey.

"A whole loaf of bread was tooken last night," said Dinah. "It was tooken right out ob de bread box," she went on, "and I'se shuah it wasn't no rat, fo' he couldn't open my box."

"I don't know," said Mrs. Bobbsey. "Rats are pretty smart sometimes."

"They are smart enough to keep out of my trap," said Papa Bobbsey. "I must set some new ones, I think."

"Well, I don't think it was any rat," said Dinah, as she went on serving breakfast.

There was so much to do that day, and so much to see, that the Bobbsey twins, at least, and their cousins, paid little attention to the story of the missing loaf of bread. Bert did say to Harry:

"It's too bad we didn't watch last night. We might have caught whoever it was that took the bread."

"Who do you think it was?" asked Harry.

"Oh, some tramps," said Bert. "It couldn't be anybody else."

They went ashore after breakfast, close to the waterfall.

"Papa, you said you would show us where we could walk under the water without getting wet," Nan reminded him.

"Oh, yes," said Mr. Bobbsey. "I have never been to these falls, but I have read about them." Then he showed the children a place, near the shore of the lake, where they could slip in right behind the thin veil of water that fell over the black rocks, high above their heads. Back of the falling water there was a space which the waves had worn in the stone. It was damp, but not enough to wet their feet. There they stood, behind the sheet of water, and looked out through it to the lake, into which it fell with a great splashing and foaming.

"Oh, isn't this wonderful!" cried Nan.

"It surely is," said Dorothy, with a sigh. "I never saw anything so pretty."

"And what queer stones!" cried Bert, as he picked up some that had been worn into odd shapes by the action of the water.

The Bobbseys spent some little time at the waterfall, and then, as there was a pretty little island near it, where picnic parties often went for the day, they went there in the Bluebird, going ashore for their dinner.

"But I'm not going to play Robinson Crusoe again," said Freddie, as he remembered the time he had been caught in the cave.

At the end of a pleasant day on the island, the Bobbseys again went on board the houseboat for supper.

"We'll watch sure to-night," said Bert to Harry, as they got ready for bed. "We won't go to sleep at all."

"All right," agreed the country cousin.

It was hard work, but they managed to stay awake. When the boat was quiet, and every one else asleep, Harry and Bert stole softly out of their room and went to the passageway between the dining-room and kitchen.

"You watch from the kitchen, and I'll watch from the dining-room," Bert told his cousin. "Then, no matter which way that rat goes, we'll see him."

"Do you think it was a rat?" asked Harry.

"Well, I'm not sure," his cousin answered. "But maybe we'll find out to-night."

"We ought to have something to hit him with, if we see a rat," suggested Harry.

"That's right," Bert agreed. "I'll take the stove poker, and you can have the fire shovel. Now keep very still."

The two cousins took their places, Bert in the dining-room, and Harry in the kitchen. It was very still and quiet on the Bluebird. Up on deck Snap, the dog, could be heard moving about now and then, for he slept up there.

Bert, who had sat down in a dining-room chair, began to feel sleepy. He tried to keep open his eyes, but it was hard work. Suddenly he dozed off, and he was just on the point of falling asleep, when he heard a noise. It was a squeaking sound, as though a door had been opened.

"Or," thought Bert, "it might be the squeak of a mouse. I wonder if
Harry heard it?"

He wanted to call out, in a whisper, and ask his cousin in the dining-room, just beyond the passage. Bert could not see Harry. But Bert thought if he called, even in a whisper, he might scare the rat, or whoever, or whatever, it was, that had caused the mystery.

So Bert kept quiet and watched. The squeaking noise of the loose boards in the floor went on, and then Bert heard a sound, as though soft footsteps were coming toward him. He wanted to jump up and yell, but he kept still.

Then, suddenly, Bert saw something.

Standing in the dining-room door, looking at him, was a boy, about his own age—a boy dressed in ragged clothes, and in bare feet, and in his hand this boy held a piece of bread, and a slice of cake.

"You—you!" began Bert, wondering where he had seen that boy before. And then, before Bert could say any more, the boy turned to run away, and Bert jumped up to catch him.