  A gravitational machine is defined as an arrangement of gravitating masses
from which useful energy can be extracted. It is shown that such machines may
exist if the masses are of normal astronomical size. A simple example of a
gravitational machine, consisting of a double star with smaller masses orbiting
around it, is described. It is shown that an efficient gravitational machine
will also be an emitter of gravitational radiation. The emitted radiation sets
a limit on the possible performance of gravitational machines, and also
provides us with a possible means for detecting such machines if they exist.
