  We propose a new method to measure the mass of large-scale filaments in
galaxy redshift surveys. The method is based on the fact that the mass per unit
length of isothermal filaments depends only on their transverse velocity
dispersion. Filaments that lie perpendicular to the line of sight may therefore
have their mass per unit length measured from their thickness in redshift
space. We present preliminary tests of the method and find that it predicts the
mass per unit length of filaments in an N-body simulation to an accuracy of
~35%. Applying the method to a select region of the Perseus-Pisces supercluster
yields a mass-to-light ratio M/L_B around 460h in solar units to within a
factor of two. The method measures the mass-to-light ratio on length scales of
up to 50h^(-1) Mpc and could thereby yield new information on the behavior of
the dark matter on mass scales well beyond that of clusters of galaxies.
