  Tidal tails composed of stars should be unstable to the Jeans instability and
this can cause them to look like beads on a string. The Jeans wavelength and
tail diameter determine the wavelength and growth rate of the fastest growing
unstable mode. Consequently the distance along the tail to the first clump and
spacing between clumps can be used to estimate the mass density in the tail and
its longitudinal velocity dispersion. Clumps in the tidal tails of the globular
cluster Palomar 5 could be due to Jeans instability. We find that their spacing
is consistent with the fastest growing mode if the velocity dispersion in the
tail is similar to that in the cluster itself. While all tidal tails should
exhibit gravitational instability, we find that clusters or galaxies with low
concentration parameters are most likely to exhibit short wavelength rapidly
growing Jeans modes in their tidal tails.
