  COMPASS, a new fixed target experiment at CERN, aimed at the study of nucleon
spin structure and hadron spectroscopy, has started to collect physics data in
Autumn 2001. This paper describes the COMPASS apparatus and the measurement of
the gluon polarisation $\Delta G /G$. The apparatus consists in a solid state
polarised target and a two stage spectro-meter with high resolution tracking,
particle identification and calorimetry, capable of standing high event rates.
COMPASS measures the longitudinal spin asymmetry of open charm production in
polarised deep inelastic muon nucleon scattering: this asymmetry is directly
related to $\Delta G$ since at COMPASS energies open charm is essentially
produced by photon gluon fusion only. A second channel, used to access $\Delta
G$ with higher statistics, is the production of correlated high $p_T$ hadron
pairs.
