  We present an intermediate resolution, high S/N spectrum of the z = 2.7268
galaxy MS 1512-cB58, obtained with LRIS on the Keck I telescope and covering
the rest frame far-UV from 1150 to 1930 A. Gravitational lensing by a
foreground cluster boosts the flux from cB58 by a factor of about 30 and
provides the opportunity for a first quantitative study of the physical
properties of star forming galaxies at high redshift. Our main results are as
follows. The ultraviolet spectral properties of cB58 are remarkably similar to
those of nearby star forming galaxies. The P-Cygni profiles of CIV and NV are
best matched by continuous star formation with a Salpeter IMF extending beyond
50 solar masses---we find no evidence for either a flatter IMF (at the high
mass end), or an IMF deficient in the most massive stars. There are clues in
our data that the metallicity of both the stars and the gas is a few times
below solar. Our best estimate, approximately equal to 1/4 solar, is 3 times
higher than the typical metallicity of damped Lyman alpha systems at the same
redshift, consistent with the proposal that the galaxies which dominate the H I
absorption cross-section are generally forming stars at a slower rate than L*
Lyman break galaxies like cB58. The relative velocities of the stellar lines,
interstellar absorption, and H II emission indicate the existence of
large-scale outflows in the interstellar medium of cB58, with a bulk outward
motion of 200 km/s and a mass loss rate of approximately 60 solar masses per
year, roughly comparable to the star formation rate. Such galactic winds may
well be the mechanism which self-regulates star formation, distributes metals
over large volumes and allows the escape of ionizing photons into the IGM.
