  We examine the line-of-sight clustering of QSO heavy-element absorption-line
systems, using a new measure of clustering, called the reduced second moment
measure, that directly measures the mean over-density of absorbers. While
closely related to other second-order measures such as the correlation function
or the power spectrum, this measure has a number of distinct statistical
properties which make possible a continuous exploration of clustering as a
function of scale. From a sample of 352 C IV absorbers with median redshift
2.2, drawn from the spectra of 274 QSOs, we find that the absorbers are
strongly clustered on scales from 1 to 20 Mpc. Furthermore, there appears to be
a sharp break at 20 Mpc, with significant clustering on scales up to 100 Mpc in
excess of that which would be expected from a smooth transition to homogeneity.
There is no evidence of clustering on scales greater than 100 Mpc. These
results suggest that strong C IV absorbers along a line of sight are indicators
of clusters and possibly superclusters, a relationship that is supported by
recent observations of ``Lyman break'' galaxies.
