If you look from a distance, you observe a sea of roofs, and have no more knowledge of the dark streams of people than of denizens of some unknown ocean. But the city is always a heaving and restless place, with its own torrents and billows, its foam and spray. The sound of its streets is like the murmur from a sea shell and in the great fogs of the past the citizens believed themselves to be lying on the floor of the ocean.