The peculiar velocity of the Local Group- II. H I observations of SC galaxies.

HI observations of a sample of 163 Sc galaxies made with the Mk IA and Mk II radio telescopes at Jodrell Bank are presented. These observations complete the northern part of a survey of intermediate-type spirals for the purpose of establishing the peculiar velocity of the Local Group. The overall rms error in redshift determination is 5 km s^-1^ and the rms error in velocity width determination is 10 km s^-1^. An analysis of the inclination-dependent properties of Sc galaxies shows that edge-on Sc galaxies are only 17+/-7 per cent larger in apparent diameter than face-on Sc galaxies compared with the Second Reference Catalogue value of 46+/-11 per cent (for an axial ratio of 0.20). Sc galaxies also appear to have high internal obscuration and may be optically thicker in blue light than earlier-type spirals. No significant variation in HI self-absorption with inclination is found. Optical, infrared, far-infrared and radio observations are used to demonstrate that the global properties of Sc galaxies are adequately described in terms of three uncorrelated parameters which can be related to linear diameter, quiescent star-formation rate and embedded starburst-type activity. This three-parameter sequence is effectively an orthogonal three-dimensional classification system which is able to account for the global properties of Sc galaxies with an accuracy close to the limit of measurement error. It is suggested that the scheme may extend to other types of spiral galaxy.