An ambitious Mark Twain Festival begins tonight on public television with a two-hour dramatization of "The Innocents Abroad." Viewers who are unsure what the Pyramids, the Grand Canal and a batch of foundation money look like will have their curiosity sated, probably within the first few minutes, while others (who operate on the theory that public television is bound to be good for you) will sit patiently till the last dog is hung.

It's clear enough what is intended in the show--a summer-day tone, full of agreeable-looking people traveling abroad to see the sights, with a bit of humor here, a small chuckle there, and nothing to upset a nervous digestion.

It is certain the director was sunstruck in his early years by Mickey Rooney films, but thought they were a trifle too tense. Tonight's program (on Channel 26 at 9) is slack and relaxed and is modeled on a hammock at 4 p.m. on an August day.

It has the merit, then, of being utterly unlike anything else ever likely to be seen on television, since the actors take their own sweet time delivering their lines, and this may be the place to say that Mark Twain's lines are not very good to begin with; indeed, his book is rather a poor effort, although it was well received in 1869 when it was published.

Twain, at the age of 31, went along on a tour of Europe, the Holy Land and Egypt with a group of tourists. He wrote travel letters for an obscure California newspaper. His chatty style does not lend itself well to dramatic treatment.

Such is the brassy gall of public television that when the next fund-raising month comes along we will probably be reminded that it was not on any commercial network that we saw "The Innocents Abroad," and that will indeed be true. It is unfortunate that public television so often raises hallelujahs about shows that no self-respecting money-grubbing commercial outfit would touch with a pole, and "The Innocents Abroad" is a pretty example of tedious time-wasting.

The actors, Craig Wasson, Brooke Adams, David Ogden Stiers and a handful of others, slowly wend their way through the general curfew, while Luigi Proietti as a tour guide is pretty lively and alert, like a merry tunester at a funeral.

It is not likely--in the sense that it does not seem possible--that the three remaining shows in this Twain festival on public television will be so dull. They will encore (presumably they have already been seen) as follows: "Life on the Mississippi" on May 16, "The Mysterious Stranger" on May 23 and "The Private History of a Campaign That Failed" on May 30.

There is a theory that good writers should be approached with extreme caution and only after a long softening-up process. Thus it is thought kids should read comic books about Shakespeare, the Bible and so forth, and then should meet these great works again in kindergarten where they play Lear, Cleopatra, Job and the rest in informal class games. Later on they are exposed to movies about these characters and finally, about the age of 30, they may gingerly dip into the written originals themselves.

This approach to good books may have much to do with the widespread perception that Americans under the age of 47 are nitwits. In any case, tonight's show is a dandy example of the general theory that words in a book may bite you or be bad for your health, and that you should get to a writer like Mark Twain by feeble steps and slow.

If anybody, however, is reckless enough to do something daring, he might try reading "Huckleberry Finn," by Mark Twain, without any advance preparation at all, and if the shock of real words by a real writer does not kill the reader after tonight's television drama, he might see why Twain is reckoned among our best novelists. He will certainly be astounded at the difference between a careful professional effort and a television plot.