In this paper we present a detailed analysis of a number of quiescent prominences for which the components of the magnetic field as well as the electron density and emission measure were previously obtained from quasi-simultaneous measurements in hydrogen Hα and helium D3 lines. From magnetic equilibrium models of the Kippenhahn–Schlüter type one can calculate the gas pressure, density, column mass and geometrical width. The same set of physical parameters can also be derived from a NLTE hydrogen-line analysis. We have studied the mutual correlations between these two sets of parameters. Very large differences, reaching an order of magnitude, were found between these two sets, both for individual prominences and for the mean values over all prominences used in this investigation. Finally, we discuss some implications of our results.