  Gravitational interactions were studied by measuring the influence of small
external field masses on a microwave resonator. It consisted of two spherical
mirrors, which acted as independent pendulumsindividually suspended by strings.
Two identical field masses weremoved along the axis of the resonator
symmetrically and periodically betweena near and a far position. Their
gravitational interaction altered the distance between the mirrors, changing
the resonance frequency, which was measured and found consistent with Newton's
law of gravity.
  The acceleration of a single mirror caused by the two field masses at the
closest position varied from $5.4 10^{-12} m/s^2$ to $259 \cdot 10^{-12}\
m/s^2$.
