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 BOOT'S BLO WER. 41 1

sufficient if they subtend the pitch angle, or angle included between one pair of teeth. With such wheels as those of Fig. 1, for instance, they need not extend beyond s and t ; while if they be made semi-cylindrical the wheels need have two teeth only. This form, as shown in Fig. 2, is that adopted in Fabry's later wheels. Epicycloidal profiles are again adopted for the faces o p, q, r, etc. ; and at m n there is contact between the central arm of the wheel b and the root-cylinder or boss of a. The space between the latter and the cylindrical sides of the chamber is the tooth-ring cylinder, the volume of which again approximates very nearly to the volume of air delivered per revolution. Fabry's ventilators are constructed of from 3 to 4 metres diameter and 2 to 3 metres breadth, and move comparatively slowly, namely, at from 30 to 60 revolutions per minute.* The framework of the wheels is mostly made of wood, tin plates being nailed upon it at the places of contact mn, etc., so that the whole construction bears the least possible resemblance to a toothed wheel. We can there- fore easily understand how the theoretic connection between the Pappenheim machine and that of Fabry has remained unnoticed by practical men.

96-

Root's Blower. Plate XXXIV.

The blowing machine of Eoot represented in Fig. 1, PL XXXIV. was exhibited at the Paris Exhibition of 1867. f The wheels were about three feet in diameter and nearly seven feet broad ; they were driven at a great velocity, and delivered a large volume of air at a considerable pressure. The profile p n r is circular, and works continually in contact { with the profile $ m o of the other wheel.

de TExpl. des Mines de Houille ; Polyt. CentralUatt, 1858, p. 506; also Civil- Inginieur.
 * Cf. Zeitschrift des Vereins deutscher Ingenieure, vol. i., p. 1 40 ; Ponson, Traiti

t The Engineer, August, 1867, p. 146,

J I believe that now, at all events, the profiles of the wheels are made so as just not to come into contact ; it is considered that the absence of friction thus attained more than compensates for the small leakage of air which occurs at ordinary pressures and velocities.