1. Any branched ventilation networks with a different number of exits to the surface and natural drafts are calculated by the methods of mesh currents and closed meshes. 2. In the presence of homogeneous resistances in a network which have not less than two junctions the method of mesh currents is simpler since it requires setting up and solving a system with a fewer number of unknowns. 3. In the case where the ventilation network is comprised of nonhomogeneous air-flow resistances (the presence of caved zones) we can use the method of closed meshes. 4. If the magnitudes of the resistances of the branches and the quantity of air flowing along the branch are known, it is possible to establish the values of the natural drafts acting on the ventilation network.