The switcheroo    

London was fifteen hundred years old, and was a great town—for that day. It had a hundred thousand inhabitants—some think double as many.  The streets were very narrow, and crooked, and dirty, especially in the part where Tom Canty lived, which was not far from London Bridge.

From the book:

Of great interest to me, in this chapter, is the dress and behaviour of the tourists, especially their self-justified vandalism. Twain is amused by the former and offended by the later. He remarks on a somewhat related behaviour of the doctor. Once knowledge that a doctor was present in one of the Syrian villages they'd stopped in, the people flocked to him "... and upon his face was written the unquestioning faith that nothing on earth could prevent the patient from getting well now."