They imagined Dutchy's humiliation, when he should rise after a superhuman effort and find the place silent and vacant, nobody there to applaud.

They were 'so full of laugh' with the idea, that they were continually exploding into muffled cackles.
Time swept on, and presently one who was peeping through the briers, said, with surprise-- 'Why, he hasn't come up, yet!' The laughing stopped.
'Boys, it 's a splendid dive,' said one.
'Never mind that,' said another, 'the joke on him is all the better for it.' There was a remark or two more, and then a pause.

Talking ceased, and all began to peer through the vines.

Before long, the boys' faces began to look uneasy, then anxious, then terrified.

Still there was no movement of the placid water.