Question

What's the difference between "Farther" and "Further"? Are they both comparative of "Far"?

Answer 1

We use farther for physical distance; we use further for degree, space, or time.

Examples:

The moon is farther from the Los Angeles than I thought.

I will provide further details tomorrow.

Answer 2

Farther is used for distance and yes, it is comparative of "far"

For example:
How much farther is the amusement park?

Further is implies a metaphorical advancement like:
Further topics will be discussed tomorrow.

source: http://learnenglishwithpictures.blogspot.com/2014/01/farther-vs-further.html