16 Prime Movers. 



pressure; and a respirator, or apparatus capable of rapidly- 

 absorbing or imparting beat, was employed. The steam, in 

 passing off to the atmosphere, through the respirator, left a 

 large quantity of heat behind ; and this was taken up by the 

 steam which next entered the cylinder. It was asserted that, 

 with this engine, only one-twentieth of the effect was lost j but 

 it is nearly the same in principle, and is open to almost the 

 same objections, as the caloric engine, which we shall notice 

 presently. 



A belief that the crank destroys power, has caused many 

 efforts to construct rotary steam-engines ; that is, such as would 

 produce a rotary motion, by the direct action of the steam, and 

 without the medium of reciprocation, which is unavoidable in 

 ordinary engines. This belief, however, is shown, by the pro- 

 perties of the lever, to have no foundation : a certain amount of 

 force is, no doubt, lost on account of obliquity of the connecting- 

 rod ; but this loss is far more than counterbalanced by the advan- 

 tages of the crank, which gradually brings the heavy masses 

 of matter to a state of rest, or motion ; and by the diminished 

 speed it causes towards the end of the stroke, which gives 

 time for the waste steam to escape before the piston returns upon 

 it. — The first rotary steam-engine, of which we have any account, 

 was that invented, or at least described, by Hero, of Alex- 

 andria, in the second century before the Christian era. The 

 steam was made to escape from apertures, near the ends, and 

 at opposite sides, of a hollow arm which turned about its 

 centre ; the reaction against the interior of the tube, opposite 

 to the apertures, caused a rotatory motion. Its principle has 

 been applied in engines constructed by Avery, in America, 

 and by Euthven, in Edinburgh ; and it is one of those contri- 

 vances which have often been reinvented ; but it is less efficient 

 than the common engine, since the steam leaves the revolving- 

 arm with a higher velocity than that of rotation. 



In other forms of rotary engine, the steam produces a con- 

 tinuous motion, by causing vanes, etc., to revolve within a drum ; 

 but it has been found impossible to keep such surfaces steam- 

 tight. 



The attempts made to obtain a more economical, or, at 

 least, a more convenient prime mover than steam, have given 

 rise to many proposed substitutes for it ; but none of them have 

 been successful. If any machine shall supersede the steam- 

 engine, it must be "cheaper and as good," or "better and as 

 cheap." However ingenious it may be, unless it fulfils one, at 

 least, of these conditions, it has no chance of being adopted. — 

 We shall first direct attention to what has been proposed, 

 rather as improvements of the ordinary engine, than as dif- 

 ferent sources of motive power. 



