我查了一下好像，两者很多时候是INTERCHANGEABLE，似乎FURTHER用法更广。

我不太追究语法，只是听着别人说哪个就用哪个。下面是我SEARCH来的信息。似乎FURTHER有时不能用FARTHER代替。

The Bottom Line

Many English speakers use both farther and further to mean "more far" in space, but only further can be used for non-physical distance and to mean "more." If you are talking about actual distance, you can usually use either word without raising any eyebrows, but keep further in mind for all of your English furthering efforts.