THE OSCILLATING ENGINE. 297
turns about 2, which was formerly the crank-pin ; the block c oscillates about the point 3, and causes the slide to turn about the same point in addition to folio wing the motion of the crank. \Ve shall call the mechanism a swinging-block (slider-crank). It will be remembered that we can reverse any of the lower pairs ( 16) without altering their relative motions*; by choosing the arrangement of slide and block shown in Fig. 218, therefore, we do not alter the mechanism. In this form it is exceedingly well known, although not so constantly employed as (C" 3 'P J -) d . A
FIG. 217.
familiar illustration is the oscillating engine, of which Fig. 218 at once reminds us. Here the slide d, in the shape of the piston, is the driving link, the special formula be'.ng therefore (OJ P-^K . There have been various attempts to elucidate the connection between the mechanisms of the oscillating and the common direct- acting steam-engine, the explanations being generally founded on some process of altering the relative dimensions of their parts.
FIG. 218.
It has been said, for instance, that the former is simply the direct- acting engine with the length of its connecting rod reduced to zero, and at the same- time, in order that motion may be possible, with its cylinder made so that it can oscillate about an axis. There is here obviously something more than a mere alteration of dimensions, and the whole process remains indistinct. We see now how entirely different and at the same time how completely
