The radial velocity field of H I in M33 is used to derive a rotation curve out to a radius of 6 kpc and to demonstrate that large scale deviations from circular motion within 4 kpc of the nucleus are less than 3 km s −1 . The overall agreement between the present H I and earlier H α radial velocities is satisfactory. A model of the distribution of mass indicates that all present data are consistent with a mass-to-light ratio of 5, almost independent of radius in the galaxy, a total mass M out to 6 kpc of |0.92×10¹⁰M_| and a ratio MHI / M of 0.10 within this radius. The thickness of the H I layer in the disk increases from 300 pc near the nucleus to 800 pc in the outer parts. A search for motions associated with a possible large scale density wave in the H I failed to detect any with an amplitude greater than 3 km s −1 .
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Warner et al. (1973) studied this question.