The bell in the bridge

When Charlie visits his hardworking grandparents in the summer, he often is left to himself, and he is lonely. So he goes out to play by the stream, with a tin can for tadpoles, a special weed-whacking stick, and stones to drop from the iron bridge. One day he notices that when he strikes the bridge with a big stone, it rings with a bong like a church bell and echoes into the valley. And sometimes a faint, very distant, different-sounding bong comes back. Is it an echo of an echo? Or could someone else, like him, be ringing another bridge altogether?