SECT, v.] NONCONDENSING ENGINES. 181 



nothing in the course of the stroke : for some objects this variation is desirable, 

 because the motion of the piston is not accelerated so much towards the end of the 

 stroke. It may be used for any of the purposes to which steam power has been 

 found applicable ; and where water is not easily procurable, it becomes the most. 

 economical species of engine. 



381. Double cylinder expansive engine. An engine of the noncondensing kind 

 may be worked expansively by means of a double cylinder, according to Horn- 

 blower's method, (art. 32.) In Plate iv. Fig. 5. C is the cylinder for the strong 

 steam, and B that in which it acts by expansion. The steam enters at S from the 

 boiler, and passing through the passage t at the top of the small cylinder, forces 

 down the piston ; the steam previously in the cylinder C passes through b, and 

 ascending by the pipe e, enters the large cylinder B at a, and by its expanding 

 force causes the piston to descend, the expanded steam below the piston escaping to 

 the atmosphere by the passage c and through d. The pistons in the passages, 

 being moved by the rods g, ft, to the other sides of the apertures to the cylinders, 

 the pressures are reversed, and the expanded steam escapes to the atmosphere by 

 the passage a, through the aperture /. This construction is not very complex 

 to obtain the motion of both pistons in the same direction ; but it obviously could 

 be done by one slide if the pistons had contrary motions, and I see no sound 

 objection to their motions being contrary; and then the axis of motion should 

 be between them. The effect of this mode of applying steam is the next point of 

 consideration. 



382. Let /be the force in inches of mercury or 0-385 /the force in Ibs. on 

 a circular inch of the small piston, a its area, and / the length of its stroke. Also, 

 let m a be the area of the large piston, and n I the length of its stroke. Then at 

 any portion x of the descent of the piston in the small cylinder, nx = the descent 

 in the large one. The original space of the steam being / a, and its pressure being 



inversely as its bulk ; (I - x) a + m n ax -. I a . . /-. / + (> f _ i) ^ = the elastic force 



of the steam between the pistons. And if "385 /' be the resistance from friction, 

 loss of force, and the resistance of the atmosphere, we have, 



385/ " - ,+(..'-1,. + ,+,,".'-.).) - - 385 - / = 



the forces of both cylinders ; and the differential of the power is, 



/ + (m n 1) x 

 Its integral is, 



385/a | x + I hyp. log. (/ + ^HT^l *) j - -385 m n af x, 



