458 THE STEAM-ENGINE. 



planation : let the point o be the centre of the great working beam, and let 

 O P' be the arm of the beam on the side of the steam-cylinder. Let P be a 

 pivot upon the beam, at the middle point between its centre and its extremi- 

 ty P' ; and let the links P /?, P' x', and P p, be joined together, as already de- 

 scribed. Let the point or pivot o be attached to some part of the fixed framing 

 of the engine or engine-house, and let the rod o p, equal to half the arm of the 

 beam, be attached by a pivot to the corner of the parallelogram at p. Let the 

 end of the steam piston-rod be attached to the corner of the parallelogram a/, 

 and let the end of the air-pump be attached to the middle point x of the link 

 P p ; by which arrangement it is evident that the rectilinear motion of the two 

 piston-rods will be rendered compatible with the alternate circular motions of 

 the points P 7 and P on the beam. 



Among the many mechanical inventions produced by the fertile genius of 

 Watt, there is none which has excited such universal, such unqualified, and 

 such merited admiration, as that of the parallel motion. It is indeed impossi- 

 ble, even for an eye unaccustomed to view mechanical combinations, to behold 

 the beam of a steam-engine moving the pistons, through the instrumentality of 

 the parallel motion, without an instinctive feeling of pleasure at the unexpect- 

 ed fulfilment of an end by means having so little apparent connexion with it. 

 When this feeling was expressed to Watt himself, by those who first beheld ( 

 the performance of this exquisite mechanism, he exclaimed, with his usual j 

 vivacity, that he himself, when he first beheld his own contrivance in action, ( 

 was affected by the same sense of pleasure and surprise at its regularity and [ 

 precision. He said that he received from it the same species of enjoyment 

 that usually accompanies the first view of the successful invention of another 

 person. 



" Among the parts composing the steam-engine, you have doubtless," says- 

 M. Arago, " observed a certain articulated parallelogram. At each ascent and 

 descent of the piston, its angles open and close with the sweetness I had al- 

 most said with the grace which charms you in the gestures of a consummate 

 actor. Follow with your eye alternately the progress of its successive changes, 

 and you will find them subject to the most curious geometrical conditions. 

 You will see, that of the four angles of the jointed parallelograms, three de- 

 scribe circular arches, but the fourth, which holds the piston-rod, is moved 

 nearly in a straight line. The immense utility of this result strikes mechani- 

 cians with even less force than the simplicity of the means by which Watt has 

 attained it." 



The parallel motion, of which there are several other varieties depending, 

 however, generally upon the same principle formed part of a patent which 

 Mr. Watt obtained in the year 1784, another part of which patent was for a 

 locomotive-engine, by which a carriage was to be propelled on a road. In a 

 letter to Mr. Smeaton, dated 22d October, in the same year, Watt says : 



" I have lately contrived several methods of getting entirely rid of all the 

 chains and circular arches about the great levers of steam-engines, and never- 

 theless making the piston-rods ascend and descend perpendicularly, without 

 any sliding motions or right-lined guides, merely by combinations of motions 

 about centres ; and with this further advantage, that they answer equally well 

 to push upward as to pull downward, so that this method is applicable to our i 

 double engines, which act both in the ascent and descent of their pistons. 



" A rotative-engine of this species with the new motion which is now at ( 

 work in our manufactory (but must be sent away very soon) answers admira- 

 bly. It has cost much brain-work to contrive proper working-gear for these 

 double engines, but I have at last done it tolerably well, by means of the cir- 

 cular valves, placed in an inve-*ed position, so as to be opened by the force of 



