[The following was written in March, 2002.  Beware spoilers at end.]

I just finished _On_ by Adam Roberts.

Except for the reasons listed below I would have strongly recommended
this book.

I liked the writing and there is enough going on to hold my interest.
(I currently have a limited attention span that makes me prefer short
stories over boring novels.)

The setting is a vertical world, "the wall", an apparently infinite
cliff with humans living on the ledges and crevices.  Although the
culture is primitive, there are mostly nonfunctional remnants of
high technology.


A question first, then discussion that will not reveal the
plot any more than the cover of the book, and finally
spoiler-laden criticism.


Is this book intended to be part of a series?  It just stops
without warning or resolution and I would have assumed the intent
was a cliffhanger ending (that's an especially apt phrase for
the world of _On_) except that there is no evidence in the book
or elsewhere that a sequel is planned or that the book is not
intended to be complete.


It is clear early on that the world is truly vertical.  The sun
rises from under the wall (as the inhabitants call the world),
so it can't be a tall cliff sticking up out of a normal world.
As far as anybody knows the people who fall off the wall continue
to fall forever, although there are rumours of a bottom.

What are the consequences of this sort of life?  One basic
result is the difficulty of travel.  There are few routes
between settlements and most of those are guarded by people
charging tolls.  (His other novel, _Salt_, also features
nasty businessmen.)

This is an SF novel, not fantasy, so my rational thinking engaged.
(The book has an appendix with a pseudo-scientific explanation of
the world, so I was on the right track.)

Having studied fluid mechanics, planetary atmopsheres, and even
basic physics, I realized there was a problem with the world.
The wall is clearly very large.  If it is finite at all, it is
many tens of kilomters tall.  So why does it have an atmosphere?
There should either be a constant, strong wind blowing downhill
or a pressure and temperature gradient.  Maybe there won't be
a temperature gradient if the air is in thermal equilibrium with
the wall (which would require that the atmosphere not extend far
out from the wall) but there should be a strong change in pressure
with height and the extent of human habitation is too large to
be consistent with such pressure changes.

So the author wanted a setting and was willing to ignore some
physics to get it.


The first two thirds of the book features our hero Tighe wandering
around getting in trouble.  ("Tighe" decomposes into "Tig" + "-he".
The "-he" suffix is used for males and "-she" for females.)

The last third is a big change.  Tighe meets a man -- or something
like a man -- who proceeds to let loose a large mass of authorial
exposition explaining the world.



SPOILERS



It turns out one can fall forever.  The wall is finite but unbounded.
In fact, the wall is Earth after a scientific experiment gone wrong
rotates the direction of gravity by 90 degrees.  Lines of gravitational
force go around the Earth instead of into it.  (The author is aware that
gravity is no longer a potential field and one can extract unlimited
energy from it.)