Far-UV sensitivity of the Cosmic Origins Spectrograph
  We demonstrate that the G140L segment B channel of the Cosmic Origins
Spectrograph (COS) recently installed on the {\it Hubble Space Telescope (HST)}
has an effective area consistent with $\sim$ 10 cm$^2$ in the bandpass between
the Lyman edge at 912 \AA and Lyman $\beta$, rising to a peak in excess of 1000
cm$^2$ longward of 1130 \AA. This is a new wavelength regime for {\it HST} and
will allow opportunities for unique science investigations. In particular,
investigations seeking to quantify the escape fraction of Lyman continuum
photons from galaxies at low redshift, determine the scale-length of the
hardness variation in the metagalactic ionizing background over the redshift
range 2 $< z \lesssim$ 2.8, measure the ratio of CO to H$_2$ in dense
interstellar environments with $A_V >$ 3, or harness the high temperature
diagnostic power of the \ion{O}{6} $\lambda\lambda$ 1032, 1038 doublet can now
be carried out with unprecedented sensitivity.
