  We construct a time dependent adjustment mechanism for the cosmological
"constant" which could be at work in a late Friedmann-Robertson-Walker universe
dominated by quintessence and matter. It makes use of a Brans-Dicke field that
couples to the evolving standard-model vacuum energy density. Our explicit
model possesses a stable late-time solution with a fixed ratio of matter and
field energy densities. No fine tuning of model parameters or initial
conditions is required.
