Ballistic bow-shock model of Ostriker et al. (2001) compared with knot chains A1 (top row), B2 (middle row), and C1 (bottom row). In the column on the left 6″ × 6″ zoom-ins of the SOUL H2 image are shown with the bow-shock model shape overlaid. In the middle column, the transverse velocities of the involved knots are compared with the mean shell (dotted line) and outer surface layer (solid line) values predicted by the model. The same comparison is made in the column on the right for the longitudinal velocities. The knots whose speed departs from the model predictions the most are marked in red and labelled. For the sake of simplicity, the outflow is assumed to lie on the plane of the sky.