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Dr. A. K. Dewdney
Scientific American
415 Madison Avenue 
New York, N.Y. 10017

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Dear Dr. Dewdney:

	The fractal mountains you describe will differ from
all real mountains in an important respect.  Real mountains are
formed by erosion, and except for rather infrequent lakes, don't
hold water, i.e. there are always paths for water to run off.  As
far as I can see, fractal mountains constructed by the algorithm
you describe will have many more lakes, and there won't be river
valleys and canyons.  It would be interesting to simulate rain
on fractal mountains and see what it looks like when all the lakes
are full.

	Note, however, that the fractal mountains look real.  This
means that there is yet another kind of optical illusion.   Evidently,
the difference between real and fractal mountains in this important
characteristic is not obvious to the human eye.  A further consequence
is that fractal objects should not be used in programs that simulate
physical phenomena without further investigation.  Although they
may look like physical objects, they probably won't behave like them.

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Sincerely,       

John McCarthy    
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JMC/ra 
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