"Look!" said Tilda, pointing to a distant ripple drawn straight across
the surface. "There goes a rat, and I've won!"
The boy said--
"A boat takes up room in the water, doesn't it?"
"0' course it does. But what's that got to do with rats?"
"Nothing. I was thinking of Sam's puzzle, and I've guessed it. A boat
going downwards through a lock would want a lock full, all but the water
it pushes out from the room it takes up. Wouldn't it?"
"I s'pose so," said Tilda doubtfully.
"But a boat going up will want a lock full, and that water too. And
that's why an empty boat going downhill takes more water than a loaded
one, and less going up."
To Tilda the puzzle remained a puzzle. "It _sounds_ all right," she
allowed. "But what makes you so clever about boats?"
"I've _got_ to know about them. Else how shall we ever find the
Island?"
She thought for half a minute.
"You're sure about that Island?" she asked, a trifle anxiously.
Arthur Miles turned to her with a confident smile.
"Of course I'm sure."
"Well, we'll arsk about it when we get to Stratford-on-Avon."
She was about to say more, but checked herself at sight of a barge
coming down the canal--slowly, and as yet so far away that the tramp of
the tow-horse's hoofs on the path was scarcely audible.
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