  A derivation of the optical axis lenght fluctations due by tilts of the
mirrors of the Fabry-Perot cavity of long-baseline interferometers for the
detection of gravitational waves in presence of the gravitational field of the
earth is discussed. By comparing with the typical tilt-induced noises it is
shown that this potential signal, which is considered a weak source of noise,
is negligible for the first generation of gravitational waves interferometers,
but, in principle, this effect could be used for high precision measures of the
gravitational acceleration if advanced projects will achieve an high
sensitivity. In that case the precision of the misure could be higher than the
gravimeter realized by the Istituto di Metrologia ``Gustavo Colonnetti''.

General Relativity and Quantum Cosmology