Read it through once

When the boys were fifteen and upward, Tom was "showing off" in the river one day, when he was taken with a cramp, and shouted for help. It was a common trick with the boys--particularly if a stranger was present--to pretend a cramp and howl for help; then when the stranger came tearing hand over hand to the rescue, the howler would go on struggling and howling till he was close at hand, then replace the howl with a sarcastic smile and swim blandly away, while the town boys assailed the dupe with a volley of jeers and laughter. Tom had never tried this joke as yet, but was supposed to be trying it now, so the boys held warily back; but Chambers believed his master was in earnest, therefore he swam out, and arrived in time, unfortunately, and saved his life.