This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
51. Fig. 13 shows the proper manner of dividing a main current into three branches so as to supply independent air flues. If the partitions A and B were absent, the greater part of the current would continue its motion until it was deflected by the side C. In that case, the flue D would receive a larger proportion of the current than either of the others, and the supply to the flue F would be quite insufficient.
A proper supply of air to each flue may be insured by adjusting the partitions A and B in such a manner that they will intercept the desired proportions of the main current before any change is made in its direction.
52. Fig. 14 illustrates the proper method of splitting up a main air-current into several smaller ones, and of subdividing these so as to supply a large number of vertical ducts. The method of proportioning the various branches so as to supply all the ducts with air at a uniform pressure, or otherwise, as desired, will be explained in a later section.