What Are Those Pieces of Paper?

Gwen Raverat, Period Piece: A Cambridge Childhood (London: Faber and Faber Limited, 1952), p. 34:

And so it was; I can remember the smell very well, for all the sewage went into the river, till the town was at last properly drained, when I was about ten years old. There is a tale of Queen Victoria being shown over Trinity by the Master, Dr. Whewell, and saying, as she looked down over the bridge: 'What are those pieces of paper floating down the river?' To which, with great presence of mind, he replied: 'Those, ma'am, are notices that bathing is forbidden.'