agglomerative coefficient in agnes (cluster)

I haven't read the book, but could anyone explain more
about this parameter? 

help(agnes) says that ac measures the amount of 
clustering structure found. From the definition given
in help(agnes.object), however, it seems that as long
as 
the dissimilarity of the merger in the final step of
the
algorithm is large enough, the ac value will be close
to 
1. So what does ac really mean?

Thank you,
Weiguang