The contribution to secular change made by the nonuniform velocity of the fluid at the surface of the earth's core in the presence of the earth's main field is described. It is shown that contributions to secular change made by temporal changes in the flow pattern of the fluid may be important. Results are discussed qualitatively in relation to dynamo theories of the earth's main field. It is also shown that surface fluid motions that can very nearly reproduce secular change also show features compatible with the generator models of Bullard, Herzenberg, Parker, and Tverskoy. The degree of compatibility found does not allow a preferred choice among these various generators.