Ever wonder when to use farther or further? The former indicates physical distance: It's farther from Los Angeles to Australia than it is from my bedroom to the kitchen (although it may not feel like it when I'm particularly tired). The latter is conceptual: The scientist went further in explaining the laws of physics than anyone else ever had.
Want an easy way to remember the difference? No one ever says, "And farthermore ..." to drive home a point. It's always furthermore.