  We introduce the cosmic momentum field as a new measure of the large-scale
peculiar velocity and matter fluctuation fields. The momentum field is defined
as the peculiar velocity field traced and weighted by galaxies, and is equal to
the velocity field in the linear regime. We show that the radial component of
the momentum field can be considered as a scalar field with the power spectrum
which is practically 1/3 of that of the total momentum field. We present a
formula for the power spectrum directly calculable from the observed radial
peculiar velocity data. The momentum power spectrum is measured for the MAT
sample in the Mark III catalog of peculiar velocities of galaxies. Using the
momentum power spectrum we find the amplitude of the matter power spectrum is
6400^{+2800}_{-1800} and 4500^{+2000}_{-1300} \Omega^{-1.2} (h^{-1} Mpc)^{-3}
at the wavenumbers 0.049 and 0.074 h Mpc^{-1}, respectively, where \Omega is
the density parameter. The 68% confidence limits include the cosmic variance.
The measured momentum and density power spectra together indicate that the
parameter \beta_O = \Omega^{0.6}/b_O = 0.51^{+0.13}_{-0.08} or \Omega =
0.33^{+0.15}_{-0.09} b_O^{5/3} where b_O is the bias factor for optical
galaxies.
