My own rule: far refers to distance, to something far away, so "farther", not "further".
Source: http://www.bartelby.com/64/C003/0121.htmlIs it the further you get in your trip, the farther you get in your book or the other way around? Many writers since the Middle English period have used farther and further interchangeably. A relatively recent rule, however, states that farther should be reserved for physical distance and further for nonphysical, metaphorical advancement. Thus 74 percent of the Usage Panel prefers farther in the sentence If you are planning to drive any farther than Ukiah, you’d better carry chains, whereas 64 percent prefers further in the sentence We won’t be able to answer these questions until we are further along in our research. In many cases, however, it is hard to see the difference. If we speak of a statement that is far from the truth, for example, we should also allow the use of farther in a sentence such as Nothing could be farther from the truth. But Nothing could be further from the truth is so common that it has become a fixed expression.
For usage in the UK and Australia, try here: www.andrew.cmu.edu/course/76-451/watts.html
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