The paper discusses the theoretical basis of a special turbo-generator ventilation system, in which the cooling air divides into several branches, and passes through the stator core radially in and out. An extended series of experiments on a full-size model, embodying this system, has lately been carried out by the Westinghouse Co. The tests are described in a paper by C. J. Fechheimer under the Title: ``Experimental Study of Ventilation of Turbo-Alternators.'' The fundamental questions in regard to the flow of air in any ventilation system are: 1. How high pressure is required to force through a certain volume of air per unit time? 2. How will the air distribute, axially and radially, in the different intake and discharge vents? 3. What will be the ``balanced state'' of flow, if several branches of air meet and divide in a tube, the intake and discharge taking place normal to the walls of the tube? These questions are given a thorough analysis, under certain simplifying assumptions, and it is shown that 1. The total pressure required for a certain volume of air per unit time is expressible by means of hyperbolic and trigonometric cotangents of a certain argument, which contains the geometrical dimensions of the air-circuit. 2. The air is distributed according to a simple hyperbolic or trigonometric sine-law. 3. The ``balanced state'' depends on the solution of a system of simultaneous transcendental equations. A method of solution as outlined, which is applicable for such cases where the arguments are small.