"What's the matter?" asked Mrs. Bobbsey, who had followed her husband into the passageway. "Snap and Snoop aren't quarreling, are they?"

"Indeed, no," answered Mr. Bobbsey. "But Snap is acting very strangely. I don't know what to make of him."

By this time Mrs. Bobbsey had come up, where she could see the dog. Snap was still standing in front of the door, growling, whining, and, now and then, uttering a low bark.

"What's the matter with him?" asked Mrs. Bobbsey. "Is he hungry?"

"Well, I guess he's always more or less hungry," her husband said, "but that isn't the matter with him now. I think perhaps he imagines he sees Dinah's ghost!" and he laughed.

"Snap, come here!" called Mrs. Bobbsey, and, though the dog usually minded her, this time he did not obey. He only stood near the door, growling.

"Why don't you open it, and let him see what's in there," said Bert.
"Maybe it's only some of those mice that made the noise," he went on.

"Perhaps it is," his father answered. "I'll let Snap have a chance at them."

As Mr. Bobbsey stepped up to turn the knob of the "locker," or closet door, there was a noise inside, as though something had been knocked down off a shelf. Snap barked loudly and made a spring, to be ready to jump inside the closet as soon as it was opened.

"What's that?" cried Mrs. Bobbsey, while Flossie and Freddie, a little alarmed, clung together and moved nearer to their mother.

"There's something inside there, that's sure," declared Mr. Bobbsey.
"It must be a big rat!"

"Mercy!" cried Mrs. Bobbsey. "A rat!"

"I'll have to set a trap," Mr. Bobbsey went on. "That rat has probably been taking the things to eat that Dinah missed—the corn-cakes and the sandwiches."

"That's right!" cried Bert. "That ends the mystery. Go for him, Snap!"

"Bow wow!" barked the dog, only too willing to get in the closet and shake the rat.

But, when Mr. Bobbsey opened the door, no rat ran out, not even a little mouse. Snap was ready for one, had there been any; but though he pawed around on the floor, and nosed behind the boxes and barrels, he caught nothing.

"Where is it?" asked Flossie.

"I want to see the rat!" cried Freddie. Neither of the smaller twins was afraid of animals. Of course, they did not know that rats can sometimes bite very fiercely, or they might not have been nearly so anxious to see one.

"I guess the rat got away," said Mr. Bobbsey, as he watched Snap pawing around in the locker, even pushing aside boxes with his nose.

"Hab yo' cotched de ghost?" asked Dinah, looking out from her kitchen.

"Not yet—but almost," said Mr. Bobbsey. "I must clean out this closet, and find the rat-hole. Then I'll set the trap. Come away Snap. You missed him that time."

The dog was not so sure of this. He stayed near the closet, while Mr. Bobbsey set out the boxes and barrels, but no rat was to be seen, nor even a mouse. And, the odd part of it was that, when everything was out of the locker, there was no hole to be seen, through which any of the gnawing animals might have slipped.

"That's funny," said the twins' father, as he peered about. "I don't see how that rat got in here, or got out again."

"Perhaps it wasn't a rat," suggested Mrs. Bobbsey.

"What was it, then, that made the noise?" asked her husband.

"I don't know," she answered. "Something might have bumped against the boat outside."

"Yes, that's so," admitted Mr. Bobbsey. "But Snap wouldn't act that way just on account of a noise."

The boxes and barrels were put back into the closet, but even that did not seem to satisfy Snap. He remained near the locker for some time, now and then growling and showing his teeth. Mr. Bobbsey looked in some of the other, and smaller, lockers, but all he found was a tiny hole, hardly big enough for a mouse.

"Perhaps it was a mouse," he said. "Anyhow, I'll set a trap there.
Dinah, toast me a bit of cheese."

"Cheese, Massa Bobbsey!" exclaimed the colored cook. "Yo' knows yo' cain't eat cheese. Ebery time yo' does, yo' gits de insispepsia suffin terrible—specially toasted cheese."

"I don't intend to eat it!" answered the twins' father, with a laugh. "I'm going to bait a trap with cheese to catch the mice. I don't care whether they get the indigestion or not."

"Oh! Dat's diffunt," said Dinah. "I'll toast yo' some."

The trap was set, but for two or three days, though it was often looked at, no mice were caught. Meanwhile, several times, Dinah said she missed food from her kitchen. It was only little things, though, and the Bobbseys paid small attention to her, for Dinah was often forgetful, and might have been mistaken.

"I really think we have some rats aboard," said Mr. Bobbsey. "There are some on nearly every boat. I have heard noises in the night that could be made only by rats."

"And Snap still acts queerly, whenever he passes that locker," said
Mrs. Bobbsey. "I'm not so sure it is a rat that made that noise,
Richard."

"No?" her husband asked. "What was it, then?"

But Mrs. Bobbsey either could not, or would not, say.

"I say, Harry," said Bert to his country cousin one day, when the Bluebird had come to anchor some distance down the lake, "let's try to get to the bottom of this mystery."

"What mystery?"

"Why, the one about the noise, and the sandwiches and cakes being taken, and Snap acting so funny. I'm sure there's a mystery on this boat, and we ought to find out what it is."

"I'm with you!" exclaimed Harry. "What shall we do?"

"Let's sit up some night and watch that closet," said Bert. "We can easily do it."

"Will your folks let us?"

"We won't ask them. Oh, I wouldn't do anything I knew they didn't want me to do without asking," Bert said quickly, as he saw his cousin's startled glance.

"But there's no harm in this," Bert went on. "We'll go to bed early some night, and, when all the rest of them are asleep, we'll get up and stand watch all night. You can watch part of the time, and when you get sleepy I'll take my turn. Then we can see whether anything is hiding in that closet."

"Do you think there is?" asked Harry.

"I'm sure I don't know what to think," Bert answered. "Only it's a mystery, and we ought to find out what it is."

"I'm with you," said Harry again.

"Are you talking secrets?" asked Nan, suddenly coming up just then.

"Sort of," admitted her brother, laughing.

"Oh, tell me—do!" she begged.

"No, Nan. Not now," said Bert. "This is only for us boys."

Nan tried to find out the secret, but they would not tell her.

Two days later, during which the Bluebird cruised about on the lake,
Bert said to Harry, after supper:

"We'll watch to-night, and find out what's, in that closet. Snap barked and growled every time to-day, that he passed it. I'm sure something's there."

"It does seem so," admitted Harry.

Mr. Bobbsey was steering the boat toward shore, intending to come to anchor for the night, when Flossie, who was standing up in front cried:

"Oh, look! Here's the waterfall! Oh, isn't it beautiful!"

Just before them, as they turned around a bend in the bank, was a cataract of white water, tumbling down into the lake over a precipice of black rocks—a most beautiful sight.