Victorian London - Advertising - pavement advertising

[nb. it is not clear exactly what this author means - may refer to mosaics? ed.]

Publicity invades even the asphalt pavement. It relies on the frequent rain and the habit the people have over here of looking down as they walk. When the weather is fine dust dulls the surface and nothing much is visible. But as soon as a shower has washed it clean the characters appear, letters blossom under your feet and you find yourself walking on gigantic posters. In this way, the stone flags of London are made as productive as a field of wheat.

Francis Wey, A Frenchman Sees the English in the Fifties, 1935