The effect of crossing times through the solar neighbourhood on the observed stellar age and metallicity distributions due to the oscillations perpendicular to the galactic plane is well known. Stars with high z-velocity are under-represented since they stay much longer far from the galactic plane than near the sun. Since the age and metallicity distributions of these high velocity stars are quite different from the age and metallicity distributions of all the stars moving at a mean distance 10 kpc from the galactic centre, a weighting procedure has to be adopted. The weight to be assigned to each star should be proportional to the ratio of the oscillation period in z, i.e. Tz, to the time spent within a sphere centered on the sun, i.e. τz. An approximate formula has been given by Woolley (1970): . The effect of such a weighting procedure is very important : the ratio Tz/Tz varies from 1 to more than A between young and old stellar populations. It has to be noted that the quadratic term of weighting (often neglected) is equal to the linear term for a |w| velocity of about 55 km/s.

The existence of a boundary layer circulation in a system composed of a galactic disk embedded within a surrounding gaseous halo, may be inferred from two physical principles governing the fluid dynamical interactions. The first concerns the presence of a vertical shear in the azimuthal flow of gas, and the localization of this shear to the vicinity of the disk. The second principle concerns the importance of viscous stresses with regard to their influence on the vertical structure of the flow throughout the disk region, and the resultant coupling of the disk to the halo. It is this viscous coupling that gives rise to a meridional circulation extending over several disk scale heights from the galactic plane. In this discussion we shall concentrate on a rather specific model system in order to establish the essential dynamical features of the theory. The details of a somewhat more general analysis are to appear in the Astrophysical Journal in the near future.

Dynamical processes strongly affect the chemical enrichment of gas in galaxies, so abundances in stars and the Interstellar medium can be used as probes of the dynamical history of the Galaxy. By way of tying together some diverse points, rather than summarizing the conference, I shall discuss some examples of connections between chemical and dynamical evolution. The first section of this paper mentions some of the well-known ways in which dynamical processes can affect chemical evolution, in order to outline a theoretical background to the use of abundances as clues to dynamics.