In the 19th century, London was the capital of the largest empire the world had ever known — and it was infamously filthy. It had choking, sooty fogs; the Thames River was thick with human sewage; and the streets were covered with mud. But according to Lee Jackson, author of Dirty Old London: The Victorian Fight Against Filth, mud was actually a euphemism. 'It was essentially composed of horse dung,' he tells Fresh Air's Sam Briger. 'There were tens of thousands of working horses in London [with] inevitable consequences for the streets. And the Victorians never really found an effective way of removing that, unfortunately.'"