108 



LECTURES TO SCIENCE TEACHERS. 



at. Models of a number of these are now on the table 

 before you. Many of them bear scarcely any external 

 resemblance to the common steam-engine, to which they are, 

 notwithstanding their dissimilarity, so closely related. "We 

 are not now concerned as to which form is absolutely the 

 best, but are only looking at them from a kinematic point 

 of view. But it is worth noticing that in very many cases 

 not only constructive but mechanical advantages have been 

 claimed for them. Their inventors have over and over again 



FIG. 22. 



claimed some mechanical gain more or less mysterious, com- 

 pared with the ordinary form of engine. Most of them, 

 moreover, have been called "rotary" to distinguish them 

 from reciprocating engines. Our method of analysis, 

 although only kinematic, has shown us not only that there 

 can be no mechanical advantage possessed by one over the 

 other, but that the word " rotary " is essentially a misnomer, 

 if it be supposed to indicate that there is any more or 

 different rotation in them than in ordinary engines. 



We shall now only notice one more of these engines. It 

 is one which has puzzled people a great deal, and with very 



