402 JAMES WATT. 
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 
ares 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- 
