402 JAMES WA.TT. 



communication. The contrary arrangement of the cocks 

 replaces all things in their primitive state, as soon as the 

 piston has regained its maximum height Thus similar 

 effects are reproduced indefinitely. 



The motive power is here, as explained above, exclu- 

 sively steam ; and the engine, except by the inequality 

 arising from the weight of the piston, has the same power 

 whether the piston be ascending or descending. This is 

 the reason why, from the moment of its appearance it was 

 justly called a double-acting engine. 



To render this new motive power of easy and conve- 

 nient application, Watt had to overcome other difficulties : 

 it was requisite to find the means of establishing a rigid 

 communication between the inflexible rod of the piston 

 oscillating in a straight line and a beam that oscillated 

 circularly. The solution which he gave of this important 

 problem is perhaps his most ingenious invention. 



Among the constituent parts of a steam-engine, you 

 have, no doubt, remarked a certain articulated parallelo- 

 gram. With each double oscillation it develops and con- 

 tracts itself, with the smoothness of motion, I had 

 almost said with the grace, that charms us in the ges- 

 tures of a consummate actor. Follow attentively with 

 your eye its various transformations, and you will find it 

 subject to the most curious geometrical conditions ; that 

 three of the summits of the parallelogram's angles describe 

 arcs of circles in space, whilst the fourth, the summit of 

 the angle that raises and lowers the piston-rod, moves 

 very nearly in a straight line. The immense utility of 

 the result strikes mechanics less than the simplicity of 

 means by which Watt obtained it.* 



* We here give Watt's words in relating the experiment of this artic- 

 ulated parallelogram (this beautiful arrangement is called parallel mo- 



