alt.energy.renewable

-> Its not particulary difficult to make machines that extract energy
-> from their surroundings.
-> A couple of good examples are Atmos clocks and crookes radiometers.
-> No particular reason why a gyroscopic machine that extracts energy
-> from the Earths rotation wouldnt work either.

Actually, there is a problem with that last one. To slow down the
earth's rotation, so as to extract energy from it, you must reduce its
angular momentum. But where can this angular momentum go, if your
machine is riding on the earth's surface? You need to be able to grab
on to something external to the earth, such as the gravitational field
of the moon or sun, which is what tides do. So using the tides to tap
the earth's rotational energy works, but using any kind of machine that
is on the earth's surface, without an external influence, won't work.

dow