The flow velocity and temperature fields of a perfect (non-viscous) fluid obeys the hydrodynamic equations: $$\partial_\mu\left\{[\epsilon_{QCD}(T(x))+P_{QCD}(T(x))]u^\mu(x)u^\nu(x)-g^{\mu\nu} 
P_{QCD}(T(x))\right\} = 0 \; ,
\label{hydro}$$ where $T(x)$ is the local temperature field, $P_{QCD}(T)$ is the QGP equation of state, and $\epsilon_{QCD}(T)=(TdP/dT -P)_{QCD}$ is the local proper energy density. The above equations apply in the rapidity window $|y|<1$, where the baryon chemical potential can be neglected. Eq.(\[hydro\]) provides the barometric connection between the observed flow velocity and the sought after ${\rm \bf P_{QCD}}$.

![ Left figure combines STAR and NA49 data  and shows that the directed sidewards flow, $v_1(y)$, is correlated over 8 units of rapidity at RHIC. At SPS collectivity is dominated by the overlapping fragmentation regions while at RHIC the nearly identical directed flow of in the fragmentation regions is shifted to $|y|>2$. Right figure shows the pseudo rapidity dependence of elliptic from PHOBOS .[]{data-label="line1figb"}](STARFlow_v1.eps "fig:"){height="0.3\textheight" width="45.00000%"} ![ Left figure combines STAR and NA49 data  and shows that the directed sidewards flow, $v_1(y)$, is correlated over 8 units of rapidity at RHIC. At SPS collectivity is dominated by the overlapping fragmentation regions while at RHIC the nearly identical directed flow of in the fragmentation regions is shifted to $|y|>2$. Right figure shows 