Directed and Elliptic Flow in 158 GeV/Nucleon $Pb+Pb$ Collisions

The directed and elliptic flow of protons and charged pions has been observed from the semicentral collisions of a 158 GeV/nucleon Pb beam with a Pb target. The rapidity and transverse momentum dependence of the flow has been measured. The directed flow of the pions is opposite to that of the protons but both exhibit negative flow at low $p_t$. The elliptic flow of both is fairly independent of rapidity but rises with $p_t$.