  We study a heavy piston of mass $M$ that separates finitely many ideal, unit
mass gas particles moving in two or three dimensions. Neishtadt and Sinai
previously determined a method for finding this system's averaged equation and
showed that its solutions oscillate periodically. Using averaging techniques,
we prove that the actual motions of the piston converge in probability to the
predicted averaged behavior on the time scale $M^ {1/2} $ when $M$ tends to
infinity while the total energy of the system is bounded and the number of gas
particles is fixed.
