I'm getting in on this discussion a little late, so perhaps someone else has already made the point that I'm about to contribute. Clemens was a good friend of John Ross Browne, who published _Etchings of a Whaling Cruise_ in 1846. Melville reviewed Browne's book in 1851 and actually twice mentioned Browne by name in _Moby Dick_ (chapters 32 & 56). Clemens saw Browne frequently while he was living in San Francisco during the mid-1860s and later again on the East Coast. In view of Clemens's association with Browne, it would seem impossible for him not to have known about _Moby Dick_, at least. A person who might know more about this matter is Tom Tenney, whose dissertation examines Browne's travel writings closely.