The Lost Art of the Great Speech: How to Write It * How to Deliver It

Further/farther

Use farther for physical distance and further for figurative distance, time, or a continuation of something other than distance. The choice is not always clear-cut, but the distinction is made by most good speakers most of the time. For example, "This subject will be examined further in a later chapter"; "The climber went farther up the mountain than anyone had gone before"; "The police decided not to investigate the matter further."