SECT, iv.] AND PROPORTIONS OF ENGINES. 157 



elastic force, yet they still require to be applied to engines of different species. 1 

 This will be done in Sect. v. and vi. ; but before I quit the illustration of general 

 principles, it will be desirable to investigate the rotary action of steam. 



OF COMPUTING THE POWER OF STEAM TO PRODUCE RoTARY MOTION. 



312. In a great variety of the cases where steam is employed, a continuous 

 circular motion is to be produced ; and it is very generally imagined that a great 

 advantage would be gained if the rotary motion were produced by the direct action 

 of steam, instead of being obtained by the intervention of moving parts, for con- 

 verting the rectilineal motion produced by steam into a rotary one. 



But the fact that every person who has attempted to produce an engine acting by 

 the rotary power of steam, has in a greater or less degree failed in rendering it 

 as effective as a reciprocating engine, makes the theoretical principle of rotary 

 action, an interesting subject of investigation. 



313. Conceive a piston, D E, to be fitted to a regularly curved vessel A B, so 

 that it may move round C, the centre of curvature of the vessel, and consequently 

 the centre of motion. Now whether the piston be moved by the force of high 

 pressure steam, or otherwise, the pressure on an inch of area of the piston will 



FIG. 16. 



C C. 

 a* 



be equal on all its parts ; that is, the pressure on an inch, at the most distant part 

 D from the centre of motion, is the same as the pressure on an inch at the 

 part E, nearest to that centre. But since the piston is constrained to move in 

 a circle, the effects of these equal pressures are as their distances from the 

 centre of motion, and limited by the effect of the pressure at the most distant 

 part D. Hence, if the effective pressure of the steam be 10 Ibs. on the inch, 

 we have, 



1 A series of tables calculated by these formulae were published in my ' Treatise on Rail Roads,' 

 p. 161166. 



