  It has been proposed that the cosmological constant $\Lambda$ might be
measured from geometric effects on large-scale structure. A positive vacuum
density leads to correlation-function contours which are squashed in the radial
direction when calculated assuming a matter-dominated model. We show that this
effect will be somewhat harder to detect than previous calculations have
suggested: the squashing factor is likely to be $<1.3$, given realistic
constraints on the matter contribution to $\Omega$. Moreover, the geometrical
distortion risks being confused with the redshift-space distortions caused by
the peculiar velocities associated with the growth of galaxy clustering. These
depend on the density and bias parameters via the combination $\beta\equiv
\Omega^{0.6}/b$, and we show that the main practical effect of a geometrical
flattening factor $F$ is to simulate gravitational instability with $\beta_{\rm
eff}\simeq 0.5(F-1)$. Nevertheless, with datasets of sufficient size it is
possible to distinguish the two effects; we discuss in detail how this should
be done. New-generation redshift surveys of galaxies and quasars are
potentially capable of detecting a non-zero vacuum density, if it exists at a
cosmologically interesting level.
