  The average flux decrement shortward the Ly$_{\alpha}$ emission, due to the
well-known ``forest'' of absorptions, has been measured in the spectra of 8
quasars. Quasi-simultaneous optical and IUE observations of the two low
redshift quasars PKS 0637--75 (z=0.654) and MC 1104+16 (z=0.632) have been
carried out, obtaining relatively high S/N, spectrophotometrically calibrated
data on their energy distribution from the rest frame H$_{\beta}$ to the Lyman
continuum. Six more quasars in the redshift range 2.5-3.4 have been observed in
the optical domain. For all the quasars the ``intrinsic'' continuum slope and
normalization have been estimated longward the Ly$_{\alpha}$ emission and
extrapolated towards the Lyman continuum to measure the average depressions,
which have been compared with the model statistics of the Ly$_{\alpha}$ clouds.
When all the known classes of absorbers are taken into account with plausible
values for their equivalent width distribution and evolution, a good agreement
is obtained with the observations. The results for the observed continuum
decrement at $z \sim 0.65$ are identical to those predicted by the evolution
with redshift of the number of Ly$_{\alpha}$ forest systems including the HST
data and within $2\sigma$ of the predicted value using the ``standard''
Ly$_{\alpha}$ evolution (as determined only at high z).
