Farther vs. Further

Strictly speaking, farther is used to express physical distance, while further is used to express figurative or abstract distance. It is acceptable to use further in place of farther (this usage is very common in British English), but don’t use farther to refer to figurative distance.

farther down the road (for a physical road only, not a metaphorical road)
further down the road

farther along in my studies
further along in my studies
prevent further deterioration