  We used the smallest entrance aperture (0.03 arc-sec wide slit) and highest
resolution echelle gratings (E140H and E230H) of STIS on HST to record the
interstellar absorption features for 10 different multiplets of neutral carbon
at a wavelength resolving power of 200,000 in the UV spectra of 21 early-type
stars. We measured the amount of C I in each of its three fine-structure
levels, so that we could determine nT of the absorbing gas and how much it
varies in different regions. To obtain internally consistent results for all
multiplets, we had to modify the transition f-values so that generally weak
transitions were stronger than their published values. The fine-structure
populations indicate that the median thermal pressure for our entire sample is
p/k=2240 cm^-3 K, or slightly higher for temperatures that differ appreciably
from 40 K. About 15% of the gas moving at peculiar velocities shows p/k > 10^4
cm^-3 K, and this fraction drops to 1.5% for quiescent gas. Often, there are
small amounts of gas at p/k > 10^5 cm^-3 K. These ubiquitous wisps of high
pressure material probably arise from small-scale density enhancements created
by converging flows in a turbulent medium. The C I excitations indicate a
barytropic index gamma_eff > 0.90 when the gas is compressed, which is larger
than the value 0.72 expected for material in thermal equilibrium. This
indicates that the regions have a dimension r < 0.01 pc which allows them to
compress very quickly and nearly adiabatically. Appendices of this paper
present evidence that STIS can indeed achieve R = 200,000, and that telluric
oxygen absorption lines are broadened by macroscopic motions.
