  We describe the statistical mechanics of a new method to produce very cold
atoms or molecules. The method results from trapping a gas in a potential well,
and sweeping through the well a semi-permeable barrier, one that allows
particles to leave but not to return. If the sweep is sufficiently slow, all
the particles trapped in the well compress into an arbitrarily cold gas. We
derive analytical expressions for the velocity distribution of particles in the
cold gas, and compare these results with numerical simulations.
