  We present the results of ultraviolet spectral monitoring of the Seyfert
galaxy NGC 4151 performed by the Hopkins Ultraviolet Telescope during the
Astro-2 mission. We show how the response of the Lyman continuum optical depth
to variations in the continuum flux may be used to derive estimates of the
distance of the absorbing gas from the central object. In this case, it seems
that the absorbing gas is located $\leq 50$~pc away from the central source.
