  The absorption system at $z=3.572$ towards Q1937--1009 provides the best
extragalactic measurement of the atomic deuterium to hydrogen ratio, D/H. We
have obtained a new low-resolution, high signal-to-noise ratio (SNR) Keck
spectrum to re-measure the total neutral hydrogen column density N(H I) using
the amount of Lyman continuum absorption. We develop a new method to directly
determine the intrinsic unabsorbed quasar continuum from low-resolution spectra
of the \Lya forest for the first time. We use three types of spectra to measure
N(H I): (1) A wide slit spectrum for flux calibration, (2) a high-resolution
spectrum to determine the unabsorbed continuum between \Lya forest lines, and
(3) a high SNR spectrum to measure the residual flux below the Lyman limit. We
measure Log [N(H I)] = 17.86 $\pm$ 0.02 \cm2, which is reliable because N(H I)
is fully specified by the data. This result is consistent with the N(H I)
measured by Tytler, Fan & Burles (1996) from the Lyman series absorption lines,
but not with absorption models proposed by Wampler (1996) nor estimates of the
total N(H I) by Songaila et al. (1997), both of which suggested lower N(H I)
and higher D/H. The Wampler models predict abundant flux below the Lyman limit
which is absent from both our old and new spectra, taken with different
instruments. The new Keck data is consistent with the data presented by
Songaila et al. (1997). The results differ due to the different methods of
analysis, and our measurement of the QSO continuum does not agree with the
continuum models assumed by Songaila et al. (1997).

Astrophysics