F ABETS VENTILATOR. 40
or coal-gas, if their pressure be small. It is indeed capable of a greater variety of useful applications than often exists for one and the same machine.
Fabry's Ventilator. Plate XXXIII
This well known machine is a chamber-wheel train used for a "wind pump" or ventilator. The Belgian engineer whose name it bears has introduced it with great success as a suction ventilator for mines, and is still occupied in improving it. Fig. 1 shows the profile of the wheels first used by Fabry.* The pump wheels a and I are here three-toothed, the profiles of the teeth at m n and m l n^ being epicycloids upon the pitch circles, or their equidistants. At op the profiles touch on both sides of the centre line until m and n or m : and n come together. The stream of air is therefore prevented from passing between the wheels, although the point of contact does not, as in the Pappenheim wheels, pass continuously through the whole profiles. The hollowing out of the teeth entails, however, the consequence that as each tooth leaves contact a small quantity of air is carried back to the suction-pipe. If we imagine the teeth to have been first arranged for continuous contact and then hollowed out, the capacity of the hollows thus made would give us exactly the quantity of air returned. The condition therefore remains, that the quantity of air delivered per revolution is very ap- proximately equal in volume to the tooth-ring cylinder. Thus the hollowing of the teeth does not alter the quantity of air delivered ; it prevents, however, the complete uniformity of the delivery, for the return of air takes place at intervals and not continuously. This want of uniformity might be a serious dis- advantage if the machine were working with a considerable water- pressure, but for the purposes of a ventilator, especially where the velocity is small and the pressure low, it has little appreciable influence.
It is not necessary that the recesses in the chamber should be semi-cylindrical in order to insure the joint between them and the points of the teeth being kept for a sufficiently long time ; it is
- Laboulaye, Cintmatique, Second Edition, p. 793.