Clemens bought a typewriter in 1874 and wrote practice letters to W. D.
Howells and Orion Clemens on Dec. 9.  (Fishkin, _Lighting Out_,
pp.174,244)

"Portions of _Life on the Mississippi_ were submitted to the publisher
typed" (Fishkin, 174).  Clemens did not type them himself; they were
transcribed from his longhand.  Horst Kruse "suspect[s] that the
typescript was desirable . . . to hide . . . the use of source materials,
often in the form of clippings, frequently without proper acknowledgment,
from publishers and printers" (_M.T. and "Life on the Miss."_, 134).

Clemens also had other manuscripts (_Tom Sawyer Abroad_ and _Tom Sawyer,
Detective_) typed and made revisions to the typed copy, but he did not do
the typing. (_TSA::TSD_,189-191)

Thanks, larry marshburne