338 KINEMATICS OF MACHINERY.
C(/i) that is the length of the link a, can be readily adjusted at pleasure, the piece a 2 being made very open inside to allow of this. On the face of the cross-block c there is a screw to which the chuck can be attached in the usual manner. This apparatus gives us an excellent illustration of the extraordinary way in which the constructive details of a mechanism may hide its real nature. It can easily be seen how this elliptic chuck, although so often used, has been hitherto so little understood. The way in which the very disadvantageous reduction of the chain has perpetuated itself is really extraordinary. The higher pair 2.3 wears rapidly on account of its deficiency in surface, and the motion of the mechanism becomes therefore inexact. The
FIG. 272.
arrangement shown in Fig. 247,* in which the chain is used unreduced, and to which our theoretical investigations directly led us, has not this disadvantage. It is also in other ways more convenient than the common plan, especially in its simple arrangement of the prism pairs 3 and 4 upon the back . of a disc.
We have seen that it is possible to reduce a four-linked chain to one of three links : it must in the same way be possible to reduce a three-linked to a two-linked chain under suitable circum- stances. This can be done as well with a chain previously reduced as with one containing three links in its complete form, as an illustration will show. Fig. 272 shows the chain represented in Fig. 268 reduced by another link, namely the crank a. For
- Fig. 247 represents the form which. Prof. Reuleaux has chosen for the model
of this mechanism in the kinematic collection of the Kb'nigl. Gewerbe Akademie in Berlin. A machine for cutting elliptical grooves (for man-holes etc. ), m which the mechanism was used unreduced, was exhibited at the Vienna Exhibition by a Chemnitz firm. See Record of the V. Univ. Ex. (Maw and Dredge), PI. 195.
