  A mechanical model of swimming and flying in an incompressible viscous fluid
in the absence of gravity is studied on the basis of assumed equations of
motion. The system is modeled as an assembly of rigid spheres subject to
elastic direct interactions and to periodic actuating forces which sum to zero.
Hydrodynamic interactions are taken into account in the virtual mass matrix and
in the friction matrix of the assembly. An equation of motion is derived for
the velocity of the geometric center of the assembly. The mean power is
calculated as the mean rate of dissipation. The full range of viscosity is
covered, so that the theory can be applied to the flying of birds, as well as
to the swimming of fish or bacteria. As an example a system of three equal
spheres moving along a common axis is studied.
