English: Rotary blowing fan used for blowing smiths’ fires. The fan consists of a circular casing A having the general appearance of a snail shell. Within this casing revolves a series of vanes B—in this case five—curved as shown, and attached together so as to form a wheel whose centre is a boss or hub. This boss is fixed to a shaft or spindle which revolves in bearings supported on brackets outside the casing. As the shaft is rotated, the vanes B are compelled to revolve in the direction indicated by the arrow and their rotation causes the air within the casing to rotate also. Thus a centrifugal action is set up by which there is a diminution of pressure at the centre of the fan and an increase against the outer casing. In consequence air is sucked in, through the openings C, C, at the centre of the casing around the spindle. At the same time the air which has been forced towards the outside of the casing and given a rotary motion is expelled from the opening at D.
Illustration from 1911 Encyclopædia Britannica, article Bellows and Blowing Machines