DA VIES' DISC ENGINE. 389
the chamber we can now see how the periodic sweeping of the chamber by the piston allows us to use the machine with a pressure- organ. The diaphragm E F divides the horse-shoe shaped space on each side of the disc into two parts, the volumes of which change alternately from zero to the whole capacity of the horse-shoe and back again to zero. A suitable valve gear is added and com- pletes the machine as a double acting pump or steam-engine.
I must here mention that I have used the form of a slotted cylinder at Z, for the piece which is kinematically identical with the block c, somewhat at hazard, for in the descriptions it is always spoken of simply as a packing-piece and its nature is not always indicated very clearly. In Johnson's beautiful engravings men- tioned above this important point is entirely omitted. From several of the patent drawings it might be inferred that the disc was simply provided with a radial slit as shown Fig. 3. This would be the reduction of the chain by the block c and the substi- tution for the latter of the kind of higher pairing shown in Fig. 270 ; it would be analogous, therefore, to the reduction used in Lamb's steam-engine Fig. 1, PI. XV. It may be further remarked that in the earliest disc-engines (Dakeyne's and some others) the external semicircular hoop b with the pin 3 and the external block c are omitted, the link c being thus entirely absent. The mechanism shown in our engraving, where b is the driving-link, has for its special formula (C-^-C^ .
There are considerable difficulties in the way of making a steam- tight joint at the line of contact of the disc and the cones. Bishop attempted to solve this problem by covering each of the conic surfaces with an armour of packing plates, upon which the disc could slide, and which were pressed from behind by adjustable springs. The bravado in machine-construction appears to make light of aU difficulties ! We can only wonder how far men may be led by that fascination of whatever is unusual and singular which once sur- rounded, and for many persons appears still to surround, everything connected with machinery.
If we look for the counterpart of the disc-engine among the forms of chamber-crank gear obtained from cylindric crank trains, we might take some of those formed from the turning double- slider (C 2 P-L)\ for the three infinite links of this chain corre- spond to the three right-angled links of the mechanism before us.
