The two children, left with instructions to keep near the boat and in hiding, had made a nest for themselves among the stalks of loosestrife, and sat watching the canal for sign of a moorhen or a water-rat. The afternoon was bright and very still, with a dazzle on the water and a faint touch of autumn in the air—the afterglow of summer soon to pass into grey chills and gusts of rain. For many minutes neither had spoken.
"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?"