  We discuss nuclear spin comagnetometers based on ultra-low-field nuclear
magnetic resonance in mixtures of miscible solvents, each rich in a different
nuclear spin. In one version thereof, Larmor precession of protons and ${\rm
^{19}F}$ nuclei in a mixture of thermally polarized pentane and
hexafluorobenzene is monitored via a sensitive alkali-vapor magnetometer. We
realize transverse relaxation times in excess of 20 s and suppression of
magnetic field fluctuations by a factor of 3400. We estimate it should be
possible to achieve single-shot sensitivity of about $5\times{\rm 10^{-9} Hz}$,
or about $5\times 10^{-11} {\rm Hz}$ in $\approx 1$ day of integration. In a
second version, spin precession of protons and ${\rm ^{129}Xe}$ nuclei in a
mixture of pentane and hyperpolarized liquid xenon is monitored using
superconducting quantum interference devices. Application to spin-gravity
experiments, electric dipole moment experiments, and sensitive gyroscopes is
discussed.
