as it approaches terminal speed leads to the growth of Rayleigh-Taylor instability, causing rapid breakup of the cloud. For an incompressible fluid, the Rayleigh-Taylor instability grows, in the linear regime, as ${\rm e}^{\omega t}$, where $$\omega^2={{2\pi g}\over \lambda}\left({{\rho_h-\rho_l}
\over{\rho_h+\rho_l}}\right),
\label{eq:rt}$$ $\lambda$ is the wavelength of the perturbation, and the subscripts $h$ and $l$ refer, respectively, to the heavy and light fluids (Chandrasekhar 1961, p. 428). For subsonic flows, the growth rate is similar for compressible fluids. Perturbations with the shortest wavelengths grow most rapidly, but saturate quickly when their amplitudes $A\approx\lambda$. As a result, wavelengths $\lambda\sim S$ lead most strongly to cloud breakup. For such perturbations, the above relation gives $\omega\approx17$, in fair agreement with the rate of breakup observed in the cloud, though the latter is complicated by the additional growth of Kelvin-Helmholtz instability due to the flow of gas around the cloud (cf. Murray et al. 1993).

Figure \[fig:mod1en\] shows the energy evolution of Model 1. Shown are the evolution of the total (internal, kinetic, and gravitational), the kinetic plus internal, kinetic, and internal energies. Values for the background gas are given by the solid curves, while those for the cloud are indicated by the dashed curves. The 