An answer to Tsuyoshi's question can be found in the Oxford
editon of _The Innocents Abroad_ in the afterword written
by David E. E. Sloane.  According to Sloane:

"...joke was an undisguised borrowing from Artemus Ward,
Twain's most widely known comic predecessor, who had put
the question in the mouth of a vulgar Cockney landlord
in his _Artemus Ward in London_ (1867), in reference
to Oliver Cromwell's appearing at a spiritualist's seance,
and had thus established it as a showman's joke on pretentious
manipulation of historical dignity by charlatans."
(Afterward, p. 4).

--Barb