Spectroscopy with HST/STIS Echelle and VLT/UVES of the bright QSO HE 0515 - 4414 ( z _ { em } = 1.73 ,B = 15.0 ) offers for the first time the opportunity to study the Lyman \alpha forest in the redshift range 0.9 < z < Â 1.7 at a resolution \leq 10 km s ^ { -1 } .
The number density evolution of the Lyman \alpha lines is well described by the power law approach d n / { d } z \propto ( 1 + z ) ^ { \gamma } .
We derive \gamma = 2.23 \pm 1.21 for the strong lines ( 13.64 \leq { log } N _ { { H \textsc { i } } } \leq 16.00 ) , in agreement with the Lyman \alpha forest evolution for z > 1.7 .
The expected slow-down in their evolution does not appear earlier than z \sim 1 .
For the weak lines ( 13.10 \leq { log } N _ { { H \textsc { i } } } \leq 14.00 ) we find that the HE 0515 - 4414 data for z > 1 follow the trend with \gamma = 1.40 known from z > 1.7 observations , i.e .
we confirm the difference in evolution between weak and strong lines .
We use the two-point velocity correlation function ( TPCF ) to search for clustering of the Lyman \alpha lines , yet we detect no excess in the TPCF on scales up to 10 000 km s ^ { -1 } .