. His most explicit  statement about
> the use and benign effects of tobacco, dated March 14, 1882, was  written
for
> _Study and Stimulants_, ed. A. Arthur Read, pp. 120-22.
> --
>
 I can add to this that the quote Mr. Shapiro asked about does not appear in
that 1882 letter to Read. Twain says that he began smoking at age 8, 100 a
month, was up to 200/mo by age 20, and 300/mo by age 30, and was still at
300/mo at age 46 when that letter was written. He said he'd tried to quit
several times, but it hurt his writing. He mentions ROUGHING IT as an
example, and said he was able to resume work on RI at his usual pace only
when he began smoking again. He mentioned working 5 hrs/daily for four
months each year in Elmira, 15 cigars during every 5-hour writing session
(so 3 per hour is the answer to some earlier query posed to this list). He
comments that smoking enhanced his writing and did not harm his health. Of
course, toward the very end of his life he knew he had "tobacco heart."
Amusingly, in that same letter he claimed to have little experience with
booze as a writing stimulant. Huh?

None of this answers the question posed about that quote. My recollection is
that it was part of a statement of advice he delivered in a speech to a
girls' school rather late in his career, but perhaps as early as the late
1880s.  I'm confident that MT SPEAKING holds the answer.   Hope this helps.

Kevin Mac Donnell
Austin TX