In Guns, Germs, and Steel how does Diamond define the east-west axis and the north-south axis?

The answer to this can be found on the first page of
Chapter 10 and the idea is also illustrated in Figure 10.1 on the next page.  Basically,
a north to south axis is the distance from the farthest northern point on a given land
mass to that land mass's southernmost point.  The east-west axis is the distance between
the point that is farthest east and the one that is farthest
west.

As you can see in Figure 10.1, the Americas have a
long north-south axis since it is a very long way from the northern part of Canada to
the southern tip of South America.  By contrast, the Americas have a relatively short
east-west axis since it is not all that far from the West Coast of the US to its East
Coast.