EVRARD, BAKER. 417
far as could be ascertained from a somewhat inaccessible machine, was furnished with straight radial blades instead of the epicycloidal teeth of our figure, which gave sufficiently accurate results for practical purposes, and possessed obvious constructive advantages. The return of a portion of the fluid renders the motion un-uniform, but this, as we have seen, is not a serious drawback to the efficiency of a ventilator. On the whole, therefore, it must be said that Evrard's ventilator is a very practical example of chamber- wheel gear. In order to make its delivery uniform, so as to suit it better for the purposes of a water-pump or a hydraulic engine, it is necessary only to give the teeth on I circular profiles, and to use the corresponding envelopes for the profiles of the spaces on a. The special form, however, which Evrard has chosen for his chamber-crank chain was known before his invention, a much older example of it will be described in 101. The pump con- structed on the same principle which was exhibited at Vienna was shown as the invention of Baron Greindl.*
99-
Repsold's Pump. Plate XXXVI. Fig. 1.
We have seen that the old Pappenheim invention has passed through many changes in the form and number of teeth used. Along with various alterations in the former the latter has been reduced from 6 or more to 4, 3, and even to 2. Only one step more in this really useful reduction could be made, and this has already been taken some years ago in the rotary pump made by
- In England rotary pumps have been made by Laidlow and Thomson, which
are founded upon this chamber-crank train in the form in which Evrard used it. The Engineer, May 29, 1868, p. 394. R.
Baker's "Rotary Pressure Blower" is kinematically identical with Evrard's machine, but instead of using such a profile for the spaces of a as corresponds to the relative motion of the point n, the wheel a is made a hollow drum, with a wide opening along the whole length of one only of its sides. It has therefore to revolve twice for each revolution of the fan-wheel b, while at the same time a second wheel, in every respect similar to it, has to be added in order to effect the necessary closure with the root circle of b when either of the teeth of the latter (which are here also merely thin straight blades) are moving freely, across the opening in the drum a,
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