THE DISC ENGINE. 385
which has been hitherto far less understood than any of those which have been mentioned. I mean the so-called " disc-engine," and the rotary engines and pumps constructively connected with it. Since their invention, forty years ago, the real nature of these machines has remained a riddle. It is related that a distinguished philosopher of the last decade said to his students towards the close of his life that all of them had failed to understand him except one, and that he had misunderstood him. The study of these machines reminds us irresistibly of this story, for it may really be said of them that very few have understood them, and these, their inventors included, have understood them wrongly !
There have been a number of these machines. One of them was exhibited at the Paris Exhibition of 1867; another spread its work daily all over the world by driving the Times printing-press.* Wherever we look for an explanatory description of the action of the machine, however, we find, according to my experience, that the whole matter is enveloped in a kind of glorified haziness.f The most various turnings and windings are used in the patent specifica- tions in order to make clear to others what a happy inspiration has given to the inventor ; while the theorist whose work compels him to treat the machine kinematically obviously hurries over it, some- times in a very confused manner, and sometimes even with the direct confession of failure to comprehend it.
The cause of all this lies simply in the fact that the methods of investigation hitherto used have attempted to find motions without first looking for the conditions under which they are constrained. What is really before us in the disc-engine and other machines of the same kind is nothing more than a series of chamber-crank trains formed from conic crank mechanisms. For by treating these in the way described in 78, i.e., by forming suitable links as chamber and piston and adding proper valve gear, we can obtain all those varieties of conic chamber-crank gear which have yet made their appearance.
It is remarkable that the course of empirical invention has hitherto confined itself within the limits of the case where three of the links of the chain (C^) are right-angled, the chain there- fore being (C$0 L y. This chain we called above ( 75) the
- This engine, which was no success, was thrown aside in 1857.
t See for example a passage quoted by R. S. Burn, Steam- Engine, p. 137. K CO
