THE ST1 



"ifE STEAM-ENGINE. III. 



8LIDB.VALVB WATT'H PARALLEL MOTION GOVERNOR 

 BAI 



ive now to turn our altontion to th" d:iiil>lo-acting 



which i..; much moro commonly omidoycd. In tin- 



i of this fi.ur \ulvo4 aro required viz., a titoain- 



valvo and an eduction-valve for each end of tho cylin.l. T. Tho 



upper .-tc:uii-\:ilvo :ind lhi< lower cduoticm-viilvo ;iro first opened, 



OH on the upper surface of tin- 

 a, viu-iium l.elow it. \Vhon it nearly rea< 

 thcHo aro closed, and Iho lower steam and upper 

 I'diiction-valvos opened, so that tho steam in the upper portion 

 of the cylinder U condensed, and a vacuum produced there, 



in In practice mad* mooh *maller than here ihown, the pipe* 



loading from i 10 boiler and from the two end* of UM cylinder 



ado to open lido by aide on a tme surface over which 



ni yj i.- .:..,.. .. 



*enU a model whk-h conveys e. very food idoa of 

 the conntructiou of the doublo-arting condensing engine. The 

 Blide-valvo in seen at tho left of the cylinder, the valve-rod m 

 being moved by tho eccentric z. The action of thi* will ba 

 better understood by reference to Fig. 10, which give* a detailed 

 view of them part*. 



To tho axle, A, of the fly-wheel there U fixed a circular dieo of 

 metal, E, BO arranged that it centre shall not coincide with that 

 of the axle. Surrounding tho duo there U a brass ring, c, eon- 

 nccted to the rods z, z. This ring is not fixed to the dt*o t which 



while tho lower part becomes 

 filled with steam. It will thus 

 bo seen that though there are 

 four valves to be altered, they 

 move in pairs, and thus only 

 two movements aro required. A great many plans have been 

 tried for making these, but some modification of the slide- 

 valve, the principles of which will be understood by reference 

 to Fig. 8, is now generally employed. 



In this figure c represents the cylinder, and p the piston, 

 which is just descending. 8 is the pipe by which the steam 

 enters from tho boiler, while the waste steam escapes to the 



adenser by the pipe seen below tho lower port v. D is tho 

 lide-volve, and is moved alternately up and down by tho rod 

 A, motion being imparted to this by tho eccentric. 



When it io in the position shown, the steam passes from s 

 through tho upper port, and presses on the nppe : side of tho 

 piston ; at the samo time the lower part of tl'o cylinder is open 

 to tho condenser. As tho piston comes near tho bottom of the 

 cylinder, tho slide D is pressed down as far as it can go. The 

 steam-pipo 8 is then in connection with the lower port, while 

 ie upper one communicates by the space F with the condenser, 

 and thus tho motion of the piston is reversed. The slide-valve 



155- M. 



turns in it, and thus as U 

 rotates moves the rods z,? 

 backwards and forwards. 

 This movement is imparted 

 to the bent lever s y, and 



thus conveyed to tho rod m, which moves the valves. The 

 eccentric is usually so arranged as to move the valve just 

 before the piston reaches tho end of the cylinder. Thu has to 

 be altered when the steam is employed expansively. 



We now revert to the engine shown in Fig. 9. Tho piston* 

 rod A moves the beam L. Tho clumsy arched ends formerly 

 employed aro, however, dispensed with, and the beautiful 

 mechanism known as Watt's Parallel Motion employed in their 

 place. The rods D D and E form with the end of the beam a 

 parallelogram, one corner of which is jointed to tho rod c. The. 

 other end of this rod is fixed to a pillar, B, or to the wall of the 

 building, and the lengths of the rods are so adjusted that by 

 their combined action the piston-rod is always vertical. 



In this way the alternate movement of the piston is converted 

 into an oscillating one of the beam, which is, in its torn, usually 

 changed into a rotatory motion. This is accomplished by means 

 of tho connecting rod i, and the crank K, the action of which 

 closely resembles that of the treadle of a lathe. A little thought 



