Fig. 6. We display the non-circular velocity field of the gas flow, projected onto the sky plane (v_pert), due to a slow (i.e.,=0) leading (a;left top) and trailing (b;left bottom) spiral wave; similarly we display the solutions for a fast (=100) leading (c; right top) and trailing (d; right bottom). The model galaxy disk is inclined by i=-30^o (northern side is closer to us). Thin line contours and gray scale range from -12.5 -10 to 20 by steps of 2.5 km s^-1 (dashed contours for negative values). X and Y axes are parallel to the major and minor galaxy axes (x0 westwards, y0 northwards). Isovelocities v=-100km s^-1 (dashed line) and v=100 km s^-1 (thick line) define the orientation of circular rotation in the disk (counterclockwise). The potential minimum locus is represented by the logarithmic spiral.