From a part of "Pollyanna" written by Eleanor H. Porter:
Old Tom shook his head.
"I know. I've felt it. It's nart'ral – but 'tain't best, child; 'tain't best. Take my word for it, 'tain't best." And again he bent his old head to the work before him.
I recognized "'tain't" ("It ain't" = "It is not"). However, what does nart'ral mean?
P.S. I though the novel was supposed to be children's literature, therefore it would be easy to read..
As Kate Bunting notes in a comment beneath the posted question, "nart'ral" is simply an eye-dialect representation of natural. It is far less common in Google Books search results than the alternative contraction "nat'ral," which yields dozens of matches.
In fact, Google Books searches for "nart'ral" and "nartral" for the period 1800–1980 yield only three unique matches, as follows.
From Charles Stephens, Camping Out: As Recorded by "Kit" (1872):
"I had thought like anough that mout 'a' ben the ou't," he repeated several times. "'Twas nartral anough, him bein' a bob-cat, so. But, oh!" (in a deep bass whine like a camel's) "to come 'ome 'ere—arter bein' gone amost a fotnit—an' see Beelly hung up thar" (pointing to the hook)—"dade—dade—dade—da-a-de!"
...
"Why, show!" exclaimed the old man, laying down the carcass in the doorway behind him; doing it with a touching gentleness, despite his amazement. "Why, I don't blame ye for't. 'Twas nartral anough. I make no doubt you're good-hearted yonkers as ever was."
A review of this book in The Nation (December 5, 1872) under the heading "Children's Holiday Books" indicates that the speaker is Cluey Robbins, an old man, living alone in a shanty at the edge of the Maine wilderness. The quoted excerpts describe his reaction, after having raised a wildcat from kittenhood, when he discovers that someone (one of the young men in the story who were hiking nearby) has killed it. The reviewer characterizes the old man's language as a "particular Scotch dialect," by which the reviewer seems to mean an American dialect used by Mainers of (primarily) Scottish descent. Charles Stephens himself was a native of Maine—having been born in Norway, Maine, and attended Bowdoin College in Brunswick, Maine—so presumably he had some direct exposure to the dialect he was trying to replicate.
From Rowland Robinson, Uncle Lisha's Shop: Life in a Corner of Yankeeland (1887):
"No," Solon interrupted, "fer it hain't their nart'ral climax. They hain't got abregoinese here."
...
"Those 'ere saw-whet aowls," Solon Briggs remarked, clearing his throat, "is a curiosity thing—a frik o' natur' comin' daown to her onsignificasntest teches—a nart'ral fewnonnymon, so to speak. A puffick aowl, minus the gretness of the die-mentionist kinds."
...
"The study of nart'ral hist'ry things," Solon remarked, "is a most stumenduous subjeck, cal'lated to fill the human mind of man with—er—er—ah—"
The author describes Solon Briggs at his first appearance in the novel as "the man of big if not weighty words." The story is set in the fictional township of Danvis in Charlotte (now Bennington) County in southwestern Vermont. The author was born and raised in Ferrisburgh, Vermont, near Lake Champlain. In describing the denizens of Danvis, Robinson writes:
Suffice it to say that it [Danvis] is in the state of Vermont, backed at the east by the mountains that gave the state its name, and shut out from the valley of the Champlain by outlying spurs of the same range. Thus fortified against the march of improvement, its inhabitants longer retained the primitive manners, speech, and customs of the earliest settlers of Vermont than did the population of the lake towns, whose intercourse with the great centres of trade and culture was more direct and frequent.
And from Eleanor Porter, Pollyanna (1913):
"I know. I've felt it. It's nart'ral—but 'tain't best, child; 'tain't best. Take my word for it, 'tain't best." And again he bent his old head to the work before him.
The speaker here is Old Tom, a gardener who works for Polly Harrington on her homestead in the fictional town of Beldingsville, Vermont.
All three speakers in the cited works appear to be uneducated (or marginally educated) men from from small towns or rural areas in New England. The absence of the spellings "nartral" and "nart'ral" outside New England and outside the period 1872–1913 suggest that the pronunciation being represented by these spellings was fairly localized both geographically and temporally.
