Further vs. Farther - This is another pair of words which we tend to interchange indiscriminately, but English is a language of logic and precision and we do ourselves a favor when we seek to retain or refine the structures on which the language formed. Here is the basic difference between the two, which was not codified until 1906:
Farther - refers to physical distance / spatial relationship
Further - refers to figurative distance / degree or extent
The general consensus of grammarians is that there are times when it is difficult to determine whether the distance is truly physical or figurative. In such cases, they unanimously recommend using "further" and most assert that there is no technical error in using "further" even when the distance referenced is physical. The restrictions lie more on the side of "farther" which should ONLY be used for spatial relationships.
And now...this old gospel tune, which was one of my earliest piano pieces, is firmly stuck in my head: