M. Arago's Biographical Memoir of James Watt. 261 



be liquefied in the condenser, with which it is now, in its turn, 

 in free communication. The contrary movement of the stop- 

 cocks replaces all the different parts in their first state, so 

 soon as the piston is at the summit of its elevation ; and the 

 same changes can be repeated indefinitely. The moving power 

 here, it will be observed, is the steam exclusively ; and the 

 machine, with an exception dependent upon the inequality of 

 the weight of the piston, has the same power Avhether the 

 piston ascends or descends. Hence, from its first invention, 

 it was justly designated the Double-acting Engine. 



That he might render his new motive power of ready and 

 convenient application, Mr Watt had to overcome additional 

 difficulties. He had first to discover the means of establish- 

 ing a rigid communication between the inflexible piston-rod, 

 whose stroke was perpendicular, and the beam whose move- 

 ment was circular. The solution which he gave of this im- 

 portant problem, is probably his most ingenious discovery. 



Those who have seen a steam-engine at work, have probably 

 been struck by the presence of a certain jointed parallelogram. 

 At each double oscillation it opens and closes with the smooth- 

 ness, and I had almost said the gracefulness, which so much 

 channs us in the movements of a consummate actor. Follow 

 attentively with your eye the progress of these different trans- 

 formations, and you will find them subjected to the most curi- 

 ous geometrical conditions. You will perceive that three of the 

 angles of the parallelogram describe in space certain arcs of 

 circles, whilst the fourth — the angle which raises and depresses 

 the piston-rod, — moves very nearly in a straight line. The 

 vast utility of this result, astonishes mechanicians still less 

 than the simplicity of the means by which Mr Watt obtained 

 it.* 



•' The following are the terms in which Mr "Watt gave an account of his 

 first trial of this jointed parallelogram : — " I have myself been much sur- 

 jirisfd with the regularity of its action. When I saw it in movement for 

 the first time, it afforded me all the pleasure of a novelty, and I had quite 

 the feeling as if I had been examining the inrentioii of another." Mr Smeaton, 

 who was a great admirer of Mr Watt's inventive powci"S, conceived, never- 

 theless, that to confer directly the niovcmcuts of rotation upon the a.xes, 

 would never practically become either useful or economical. He main- 

 tained also, that steam-engines would never druiu so effectually as by di- 



