  We present HST STIS observations of four quasar pairs with redshifts
0.84<z_em<1.56 and angular separation 2-3 arcmin corresponding to ~ 1-1.5
h^-1_50 Mpc transverse proper distance at z ~ 0.9. We study the distribution of
velocity differences between nearest neighbor HI Lyman_alpha absorption lines
detected in the spectra of adjacent QSOs in order to search for the possible
correlation caused by the extent or the clustering properties of the structures
traced by the absorption lines over such a scale. The significance of the
correlation signal is determined by comparison with Monte-Carlo simulations of
spectra with randomly distributed absorption lines. We find an excess of lines
with a velocity separation smaller than Delta(V) = 500 km/s significant at the
99.97% level. This clearly shows that the Lyman_alpha forest is correlated on
scales larger than 1 h^-1_50 Mpc at z ~ 1. However, out of the 20 detected
coincidences within this velocity bin, 12 have Delta(V) > 200 km/s. This
probably reflects the fact that the scale probed by our observations is not
related to the real size of individual absorbers but rather to large scale
correlation. Statistics are too small to conclude about any difference between
pairs separated by either 2 or 3 arcmin. A damped Lyman_alpha system is
detected at z_abs = 1.2412 toward LBQS0019-0145A with log[N(HI)] ~ 20.5. From
the absence of ZnII absorption, we derive a metallicity relative to solar
[Zn/H]<-1.75.
