About every usage commentator in the 20th century—from Vizetelly 1906 to Trimmer & McCrimmon 1988—has had something to say about farther and further (and sometimes farthest and furthest) as to how they should be used or how they seem to be used. Only a few venture beyond Vizetelly's original pronouncement:
Farther should be used to designate longitudinal distance; further to signify quantity or degree.
Vizetelly's dictum did not account for the educated usage of his own time or for that of the past, and it has only partially predicted usage since his time.
Farther and further are historically the same word, so it is not surprising that the two have long been used more or less interchangeably. Further is the older of the two; it appears to have originated as the comparative form of a Germanic ancestor of English forth. Farther originated in Middle English as a variant of further that was influenced by the comparative (spelled ferre)of far (then spelled fer) which it (and further) eventually replaced. So neither word was originally connected with far, but gradually they have both become so.
Henry Bradley, editor of the F volume of the OED (published in 1897), summed up what he considered standard English practice at the time. He said that farther is usually preferred as the comparative of far, that further was used where the notion of far was absent, and that there was a large intermediate class of uses in which the choice between the two was arbitrary. Fowler 1926 disagreed, seeing Bradley's description as more theoretical than actual. Fowler believed that most people did not use both terms, as they would have to do if they followed Bradley's scheme; most people made do with one or the other and the one Fowler saw as usual wasfurther. He therefore opined that further would eventually replace farther altogether. So far it has not.
What Fowler probably saw was that farther'srange of application was shrinking. This development seems to be part of a process of differentiation—a process Fowler thought would end in the demise of farther. The differentiation is most noticeable—most nearly complete— when farther and further are used as adjectives.
As adjectives, both words could at one time be used in the sense "additional":
... I have now no farther thought of danger — Thomas Gray, letter, 12 Sept. 1756
You will e'er long I suppose receive further intelligence of him —Jane Austen, letter, 11 Feb. 1801
... in that delightful state when farther beauty is known to be at hand —Jane Austen, Mansfield Park, 1814
He became aware that Mr. Jackson was clearing his throat preparatory to farther revelations —Edith Wharton, The Age of Innocence, 1920
But in present-day English further has taken over this function entirely:
"Well," he began, without any further greeting — Katherine Anne Porter, Ladies' Home Jour., August 1971
... a further volume of uncollected essays —Peter Stansky, N.Y. Times Book Rev., 1 June 1975
... no further deliveries of military equipment — Chester Bowles, Saturday Rev., 6 Nov. 1971
... hums like a tuning fork between all these fainter and further thoughts —William H. Gass, Harper's, February 1984
Farther has been relegated as an adjective to instances where either literal or figurative distance is involved:
The farther floe was pulling away in the grip of the tide —Berton Roueche, New Yorker, 22 Oct. 1966
... at the farthest remove imaginable from regional writing —Ivan Gold, N. Y. Times Book Rev., 24 Apr. 1983
And even in this function further is presenting formidable competition:
The Viking settlements had cut greater Northumbria in two, and the further part had fallen under the influence of the Celtic Highland powers —Frank Barlow, The Feudal Kingdom of England 1042-1216, 3d ed., 1972
... it was the furthest thing from everyone's mind — E. L. Doctorow, Loon Lake, 1979
... the furthest point west that Columbus reached — Graham Greene, Getting to Know the General, 1984
So for the adjective we can see that further has squeezed farther out of the "additional" sense and is giving it considerable pressure in the "more distant" sense. Fowler would probably see this situation as confirming his prediction that further would eventually win out.
As adverbs, farther and further are less well differentiated. Differentiation is most nearly complete in the "degree" sense, where there is no notion of distance. This is the use that OED editor Bradley said further was preferred for. We can find farther in this sense, but our examples are getting a bit old:
... without consulting farther with any soul living — Laurence Sterne, Tristram Shandy, 1759
Please see to it that I do not have to act any farther in the matter —Bernard DeVoto, letter, 7 June 1943
Further is now the usual choice, as Bradley predicted:
DeGaulle's violent remarks ... further strained relations —Stephen E. Ambrose, Johns Hopkins Mag., April 1966
Before she could protest further they had seized her arms and were marching her down to the boat — Daphne du Maurier, Ladies' Home Jour., August 1971
... I recommend that you have nothing further to do with this person nor with these arms transfers — Robert C. McFarlane, quoted in The Tower Commission Report, 1987
Further is also used as a sentence adverb; farther is not.
Further, I am monolingual and have no way of knowing whether a translation is faithful to the original —E. B. White, letter, 13 May 1957
I am certain, further, that she has known it could come about in just such a form —Peter Taylor, The Old Forest and Other Stories, 1985
But when spatial, temporal, or metaphorical distance is involved, farther is still thriving, pace Fowler's prediction:
... too tired, too unhappy to go farther —Bernard Malamud, The Magic Barrel, 1958
... irrigate a million acres of dry land farther downstream —John Lear, Saturday Rev., 6 Nov. 1971
I asked how much farther it was to Dublin —Renata Adler, Pitch Dark, 1983
Farther along, the obese gypsy Madame Katrinka beckons you to enter her storefront parlor —Jay McInerney, Bright Lights, Big City, 1984
Statecraft was his business, and he knew more and saw farther than they did —James A. Williamson, The Tudor Age, 1964
... has taken the effect one step farther —Susan Kenney, N.Y. Times Book Rev., 26 Jan. 1986
Cod, he pointed out, is farther down in the food chain —Garrison Keillor, Lake Wobegon Days, 1985
Nothing could be farther from the truth —Godfrey Hodgson, N. Y. Times Book Rev., 30 Jan. 1983
But further is giving farther plenty of competition for the same uses:
He could not only walk further but he walked faster —Mordecai Richler, The Apprenticeship of Duddy Kravitz, 1959
... he moved on to spread despondency further — Graham Greene, Travels with My Aunt, 1969
... 300 miles further down the river —Noel Perrin, N.Y. Times Book Rev., 6 Sept. 1981
I park my car in a better spot, further from the curve —Renata Adler, Pitch Dark, 1983
... but he got no further than Portland —Gerald Weales, Smithsonian, December 1985
... ranged still further back to Saxon and British times —John Butt, English Literature in the Mid-Eighteenth Century, edited & completed by Geoffrey Carnall, 1979
But Messrs. Lindblom and Cohen go further. They suggest that... —Andrew Hacker, N. Y. Times Book Rev., 29 July 1979
... put us one step further on the road to a police state —/. F. Stone's Bi-Weekly, 9 Feb. 1970
So in adverbial use further dominates when there is no sense of distance and as a sentence adverb, but both far-ther and further are in flourishing use whenever spatial, temporal, or metaphorical distance is involved.
Fowler would be pleased to discover that further is more commonly attested than farther in pur recent files and in the Brown University corpus (Kucera & Francis, 1967). Fowler's prediction of the demise of farther has come true only in certain uses, however. Further has all but eclipsed farther in adjective use, with farther competing only for a portion of the "more distant" use. Further has pretty well eliminated the adverbial farther from non-distance uses, and it has the sentence adverb function all to itself. But both forms are in vigorous competition in the adverbial distance uses. Fowler's prediction may be more accurate for British usage than it is for American; the British evidence in our files shows further more common than farther in all senses.