  The main result of this paper is a formula for calculating the Seiberg-Witten
invariants of 4-manifolds with fixed-point free circle actions. This is done by
showing under suitable conditions the existence of a diffeomorphism between the
moduli space of the 4-manifold and the moduli space of the quotient 3-orbifold.
Two corollaries include b_+>1 4-manifolds with fixed-point free circle actions
are simple type and a new proof that the four dimensional invariants of $Y
\times S^1$ are equal to the the three dimensional invariants of $Y$. An
infinite number of b_+=1 4-manifolds where the Seiberg-Witten invariants are
still diffeomorphism invariants are constructed and studied.
