John Merrill was a eccentric tour guide. He set up in Franconia Notch on "The Pool" where he would entertain tourists by showing them the sights and explaining his theories. Boston Rare Maps did some research and found this from the November 1867 Yale Literary Magazine:
Here is an old man in a barge, into which you enter, and he paddles you around the narrow circuit of the Pool. When you have reached the side toward the Falls, where the water is from twenty to thirty feet deep, but clear as crystal, he begins to unfold to you his favorite theory; (for you must know that, in his own estimation at least, the old man is quite a philosopher;) that the earth is a hollow sphere, inhabited on the inside, as well as the outside. He maintains his position by arguments entirely original and irrefutable; has an answer ready for every question, and seeks to proselyte you. He reads a letter he pretends to have received from Queen Victoria,-which here I insert.
We have two variations of the broadside--one containing another letter by Louis Napoleon. Come judge the validity yourself by asking for Rauner Broadside 001457 or visiting the Pool and looking into its depths.