Farther, further are often used without distinction though originally different words, farther being the comparative of far and further, in its adverbial form (as an adjective, it is without a positive), being the comparative of fore or forth.
At any rate farther basically implies a greater distance from a given point in space or sometimes in time; further implies onwardness or an advance or an addition (as in movement or progression) not only in space but in time, quantity, or degree.
In spite of this fundamental distinction in meaning, there are many occasions where it is difficult to make a choice, since the ideas of distance from a given point and of advance in movement may both be implied. In such cases either word may be used.