Local ISM density evaluation in the direction of the BS. The upper plot shows n_up versus n_low. The fitted dashed line follows n_up = 16.8 n_low + 2.3. The middle and lower plots show n_low and n_up versus the distance above the Galactic plane, z, respectively. The dashed line draws the Dickey & Lockman 1990 and Loup et al. 1993 tendencies. In all the plots, the red points are the stars in the Cygnus X region (1 ~ 79°).