CHAP. VI.] 



THE STEAM-ENGINE. 



421 



circular motion. It follows from this that the piston-rod, which 

 moves vertically, cannot be directly joined to the end of the beam, 

 because this would force it to follow the arc of a circle, and hence 

 would turn it sometimes to the right and sometimes to the left. To 

 remedy this disadvantage, which would render the engine impractic- 



FIG. 296. Principle of transmission in beam-engines. 



able, Watt invented a very ingenious system of joints, known as 

 Watt's parallel motion, of which the following is a short description. 



The piston-rod, instead of being joined directly to the extremity E 

 of the beam, is joined to the point D of the parallelogram CEDE, 

 whose four sides, though rigid and invariable in length, are jointed 



Fio. 297. Watt's jointed parallelogram. 



at their extremities, so that the angles vary according to the oscillations 

 of the beam. Moreover, the point B is attached by a rod BO to a fixed 

 point o in the immovable framework of the engine. - The relative 

 lengths of these different lines are calculated in such a way that 

 the point D describes very nearly a vertical straight line, while the 

 points c, E, B, describe arcs of circles having for their centres the 



