180 and 220 km s$^{-1}$, respectively. Since N-1 and S-1 both have about the same radial distance, we cannot confirm the expected linear trend in transverse velocity with radius predicted by eqn. 8. However, with these two data points we estimate from eqn. 9 that the ejection velocity is $490~\pm~80$ km s$^{-1}$. If we assume that we are indeed dealing with a working surface, the lack of proper motions ($\leq$160 km s$^{-1}$) allow us to estimate the ratio between the density of the medium and the density of the jet. Using the formulation of Raga, Rodríguez, & Cantó (1997), the ratio of ambient medium density, $\rho_a$, to jet density, $\rho_j$, is

$${{\rho_a} \over {\rho_j}} = \biggl({{v_j} \over {v_{ws}}} - 1 \biggr)^2,~~~~~~~~~~(10)$$

where $v_j$ and $v_{ws}$ are the jet and working surface velocities, respectively. For a jet velocity of 490 km s$^{-1}$ and an upper limit of 160 km s$^{-1}$ for the velocity of the working surface, we obtain $\rho_a/\rho_j \geq$ 4.

The precession rate of the jet is $\beta = \alpha v$. Hence with $\alpha = 2.3\pm0.4$ degree/arcsecond, and v = $490\pm80$ km s$^{-1}$, $\beta = 0.080\pm~0.02$ degrees/year. Note that the measurements of $\alpha$ and v are both 6-$\sigma$ results, 