  We show how the virial theorem can be applied to the hot gas in clusters of
galaxies to obtain a yardstick, which could then be used to determine
cosmological parameters. This yardstick relies on the assumptions of
hydrostatic equilibrium and that the gas fraction is approximately constant.
The constancy is checked empirically from a local population of clusters. By
using the observed parameters consisting of temperature, surface brightness and
radial profile $\beta$, one can calculate the expected core radius. Comparing
it to the observed angular size, one can in principle calibrate the
cosmological deceleration parameter $q_0$. We test this method on a small
sample of 6 clusters, and show its promise and accuracy. The preliminary
implications would be to suggest $q_0 \approx 0.85\pm 0.29$ with $1-\sigma$
statistical error bars, with several systematic uncertainties remaining. Taken
at face value, this would argue against a cosmological constant. The method is
robust to errors in the measurement of the core radius as long as the product
of the central density and the core radius squared $\rho_0 r_c^2$ are well
determined. New lensing and X-ray data can dramatically improve on the
statistics.
