_Ducts._--The main duct should be of metal (steel, sheet-iron, or zinc); it should be circular in shape, have as straight and short a course as possible, and be tapered in such manner that the area of cross-section at any point shall equal the combined areas of all the branch pipes which have entered it at that point (Fig. 3). Proper dimensions must be studied in relation to the size of the fan and the work to be done. Wooden ducts, unless chosen for specific reasons, such as the presence of acid in the fumes to be removed, are very unsatisfactory, as it is difficult to maintain them in an air-tight condition or to make branch pipes enter with rounded junctions. Where several branch ducts enter a main duct, situation of the fan midway between them has advantage, not only in saving metal in piping, but also in causing the distance of the fan from the farthest branch duct to be only half what it would be were the fan placed at the end of the system (see Fig. 7, p. 217). Further, the sectional area of the two collecting ducts will be less than that of one main duct, and greater uniformity of flow thereby secured. Where the two ducts join up into the single duct of the fan, the bends must be easy; otherwise the draughts would collide and neutralize one another. Branch ducts, if they cannot be made tangential to a rounded curve, should enter the main duct at an angle of 30 degrees, as by so doing equalization of the draught at different openings is made fairly uniform. The very common defect of a right-angle joint diminishes the draught by nearly one-half. Branch ducts should never be made to enter a main duct on the outer side of a bend, because at this point the pressure of the current of air inside the duct is increased. They should join up on the inside of a bend, where the pressure is reduced.