454 THE STEAM-ENGINE. 



Pig. 16. 



lion than the atmospheric engines, this might have been sufficient ; but in 

 these engines it was indispensably necessary that the piston-rod should be 

 guided with a smooth and even motion through the stuffing-box in the top of 

 the cylinder, otherwise any shake or irregularity would cause it to work loose 

 in the stuffing-box, and either to admit the air, or to let the steam escape. 

 Under these circumstances, the motion of the rack and toothed arch head were 

 inadmissible, since it was impossible by such means to impart to the piston- 

 rod that smooth and equable motion which was requisite. Another contrivance 

 which occurred to Watt was, to attach to the top of the piston-rod a bar, which 

 should extend above the beam, and to use two chains or straps, one extending 

 from the top of the bar to the lower end of the arch head, and the other from 

 the bottom of the bar to the upper end of the arch head. By such means the 

 latter strap would pull the beam down when the piston would descend, and the 

 former would pull the beam up when the piston would ascend. These con- 

 trivances, however, were superseded by the celebrated mechanism since called 

 the Parallel Motion, one of the most ingenious mechanical combinations con- 

 nected with the history of the steam-engine. 



It will be observed that the object was to connect by some inflexible means 

 the end of the piston-rod with the extremity of the beam, and so to contrive 

 the mechanism, that while the end of the beam would move alternately up and 

 down in part of a circle, the end of the piston-rod connected with the beam 

 should move up and down in a straight line. If the end of the piston-rod were 

 fastened upon the end of the beam by a pivot without any other connexion, it 

 is evident that, being moved up and down in the arch of a circle, it would be 

 drawn to the left and the right alternately, and would consequently either be 

 broken or bent, or would work loose in the stuffing-box. Instead of connect- 

 ing the end of the rod immediately with the end of the beam by a pivot, Watt 

 proposed to connect them by certain moveable rods, so arranged that, as the 

 end of the beam would move up and down in the circular arch, the rods would 

 so accommodate themselves to that motion, that the end connected with the 

 piston-rod should not be disturbed from its rectilinear course. 



To explain the principle of the mechanism called the parallel motion, let us 

 suppose that O P, fig. 17, is a rod or lever moveable on a centre O, and that 

 the end P of this rod shall move through a circular arch P P' P" P'" in a 

 vertical plane, and let its play be limited by two stops S, which shall prevent 

 its ascent above the point P, and its descent below the point P'". Let the 

 position of the rod and the limitation of its play be such that the straight line 

 A B drawn through P P'", the extreme positions of the lever O P, shall be a 

 vertical li$e. 



Let o be a point on the other side of the vertical line A B, and let the dis- 

 tance of to the right of A B be the same as the distance of o to the left of 

 A B. Let o p be a rod equal in length to O P, moving like O P on the cen- 

 tre 0, so that its extremity p shall play upward and downward through the arch 

 P P' P" P'"I * ts P* a y being limited in like manner by stops s. 



