I have actually attached the letter, published in Daily Alta California on September 16, 1867, to chapter 25. I'd like to move it to this chapter but this will take some research into Drupal modules. I don't want to upload a second copy of the letter to this site. In this letter is his description of the gunboat "set to watch the Quaker City at Leghorn", as per McKeithan.
The Innocents Abroad - Chapter 24
Mark Twain travels through Bolognia, Pistoia, and into Florence. Because of the fatigue from the journey as well as the distain he feels for Florence's treatment of Galileo and Dante, he does not speak highly of Florence. Apparently, some years in the future he does change his mind about the place as he chooses to live there for some time, at least circa 1902. He writes of the mosaics and the maltreatment received by the artists, or "pensioners". He then travels to Pisa and climbs the Leaning Tower. Twain writes of the "old original patriarchal Pendulum--the Abraham Pendulum of the world". This is the lamp hanging from the ceiling of the Baptistery that inspired Galileo. Twain then travels on to Leghorn and returns to his ship, the Quaker City. The threat of quarantine in Naples prompts two or three of the travelers to jump ship and travel to Civita aboard a French steamer.
I've again tried to create an instructive slideshow that includes images from Wikipedia Commons as well as the original illustrations from the book. The slideshow was created using the open source program Imagination. The audio was recorded live using Audacity.