THE STEAM-ENGINE. 



455 



Now, let us suppose that the ends P p of these two rods are joined by a 

 link P p, the connexion being made by a pivot, so that the angles formed by 

 the link and the rods shall be capable of changing their magnitude. This 

 link will make the motion of one rod depend on that of the other, since it will 

 preserve their extremities P p always at the same distance from each other. 

 If, therefore, we suppose the rod O P to be moved to the position O P'", its 

 extremity P tracing the arch P P' P" P" 7 , the link connecting the rods will 

 at the same time drive the extremity p of the rod op through the arch p p l p" p'", 

 so that when the extremity of the one rod arrives at P 7// , the extremity of the 

 other rod will arrive at p'" . By this arrangement, in the simultaneous motion 

 of the rods, whether upward or downward, through the circular arches to 

 which their play is limited, the extremities of the link joining them will devi- 

 ate from the vertical line A B in opposite directions. At the limits of their 



