Published online by Cambridge University Press: 14 August 2015
Observations of the neutral hydrogen in the first quadrant of galactic longitude have been analysed. The existence of large-scale streaming motions such as the streaming associated with the Sagittarius arm makes interpretation of the observations in terms of circular galactic rotation unsatisfactory. It is shown that application of the density-wave theory formulated by Lin et al. (1969) leads to a more satisfactory interpretation. Using kinematic models based on this theory the distribution and motion of the neutral hydrogen are studied. Failures of kinematic models based on circular rotation are pointed out. A map of the distribution of neutral hydrogen is produced. The Scutum arm is composed of inner and outer arcs both of which seem to be moving outward from the galactic center with velocities of the order of 30 km s−1.