  The thickness of isothermal gaseous layers and their midplane volume
densities \rho_{gas}(R) were calculated for several spiral and LSB galaxies by
solving the self-consistent equilibrium equations for gaseous discs embedded
into a stellar one. The self-gravity of the gas and influence of dark halo on
the disk thickness were taken into account. The resulting midplane volume
densities of spiral galaxies were compared with the azimuthally averaged star
formation rate SFR to verify the feasibility and universality of the Schmidt
law SFR ~ \rho_{gas}^n.
